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Volume 85, 1957-58
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The New Zealand Species of Melagraphia Gray and
Zediloma Finlay (Mollusca, Gastropoda)

[Received by the Editor, October 9, 1957.]


The shell, operculum, and external morphology of the New Zealand members of the genera Melagraphia Gray, and Zediloma Finlay are redescribed to allow for the intraspecific variation which occurs in some species.Finlay's 1927 classification is retained in the main, but Zediloma (Fractarmilla) corrosa (A.Adams) is shown to be synonymous with Z. (F.) subrostrata (Gray), and the form recently bearing this name is regarded as part of the variation of Z. (F.) subrostrata. Z. (F.) corrosa plumbea (Hutton) is shown to be a juvenile variant of subrostrata unworthy of taxonomic recognition. The mollusc known in New Zealand as Z. (F.) morio (Troschel) is regarded as part of the normal variation of Z. (F.) atrovirens (Philippi).


Suter (1913) regarded all of the species considered here to be members of the genus Monodonta, subgenus Diloma. When Finlay (1927) revised the group, he pointed out that Monodonta is a tropical form characterised by a strongly toothed columella, and that whilst the New Zealand forms agree conchologically with the South American genus Diloma they “show distinct animal features”. On the grounds that it is “conchologically aberrant”, he separated aethiops Gmelin from the others and assigned it to Melagraphia Gray, erecting a new genus Zediloma for the remainder (digna Finlay, arida Finlay, corrosa A. Adams, subrostrata Gray, atrovirens Philippi and “morio Troschel”). Zediloma s. str. was restricted to digna and arida where “the presence of a continuous nacreous band across the parietal wall, uniting both ends of the peristome, is a constant and useful feature, associated always with untoothed columella, more excavated base and spreading aperture”. The remaining four species were assigned to Fractarmilla, a new subgenus of Zediloma. The information set out in table I, much of which was unavailable to Finlay, supports his tripartite division of the group.

Final consideration of the genera and subgenera to which they are best assigned cannot profitably be undertaken until a study can be made of similar trochids, especially the Australian Austrocochlea, and perhaps the South American Diloma. Thiele (1930) gives the distribution of Melagraphia as “Einige arten im Pacificum von Neuseeland bis Japan”. Dr. Tokubei Kuroda informs me (in letter) that Thiele's statement will be based on M. piperita (Philippi) and M. radula (Philippi), and that since 1930 these species have been referred to Diloma by Wenz (1938); an arrangement followed by Kuroda and Habe (1952). Kuroda regards the Japanese neritoides and perplexa as typical Monodonta. It appears that Melagraphia may be monotypic and endemic to New Zealand.

The name Zediloma was synonymised with Melagraphia by Wenz (1938) who seems to have missed Finlay's designation of digna as the type of his new genus.

Cotton and Godfrey (1939) used the name Fractarmilla Finlay, 1927 as a subgenus of Austrocochlea Fischer, 1885 and include therein concamerata (Wood, 1828) and rudis (Gray, 1826). There is no discussion of this usage in the Australian literature Dr. D. F. McMichael referred this matter to Mr. Tom Iredale on my behalf. Mr. Iredale replied that “Australian workers do not accept Zediloma, but.

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[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

Table I. The occurrence of certain morphological features in 5 species of trochids.
Species M. aethiops Z. digna Z. arida Z. (F.) subrostrata Z. (F.) atrovirens
Band of nacre uniting ends of peristome yes yes yes no no
Columella arcuate, bluntly bidentate arcuate, smooth arcuate, smooth sub-straightened, slightly nodulous sub-straightened, slightly nodulous
Periostracum lamellose smooth smooth smooth smooth
No. of tentacles on inhalant siphon up to 30 19 14–19 16–22 17–19
Left hypo-branchial gland larger than right no yes yes no about equal
papillae between epipodial tentacles yes no no no no
course of rectum arcuate straight straight arcuate arcuate

do recognise Fractarmilla subgenerically, and the name was simply transferred to our Austrocochleas”. It has not been suggested that any of the New Zealand species belong to Austrocochlea.

The descriptions of some of the species considered in this paper are far from satisfactory, this applying with special force to the members of the subgenus Fractarmilla. Previously published figures of the radulae of all species are incomplete, lacking, or with very inadequate descriptions, and in one case referred to the wrong species.The external morphology is virtually undescribed. The key to species provided by Suter (1913) is unsatisfactory, as it does not allow for the variation present within some of the species. The descriptions given below are not based on type material, but on large collections from a number of localities in both North and South Islands.The descriptive terminology proposed by Cox (1955) has been used in the conchological descriptions Collections of shells in the Dominion. Canterbury and Otago Museums and the Geological Survey have been examined. For access to these collections I am indebted to the directors of these institutions. I also wish to thank Dr. R. K. Dell and Dr. C. A. Fleming for the loan of material and helpful discussions, and Dr. A. W. B. Powell for the loan of paratypes of Z (F) subrostrata zebrina. This work was undertaken as a necessary preliminary to a biological investigation of these species, and as such formed part of a thesis prepared in the Zoology Department, University of Canterbury. I wish to acknowledge my indebtedness to Professor E. Percival for his continued guidance and encouragement.

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Figs. 1–3.1—Melagraphia aethiops, shell, c.b chocolate coloured band. 2—Zediloma digna, shell n.b.nacreous band connecting both ends of peristome. 3—Zediloma arida, shell, a. angle at periphery of body whorl.

Key to Species

1 Basal callus pad margined by a prominent chocolate coloured band; posterior to this is an area of blue-black spotted with white (Fig. 1) Periostracum lamellose Melagraphia aethiops.
No chocolate coloured band on base, periostracum not lamellose 2.
2 With a continuous band of shining nacre (Fig. 2) connecting both ends of the peristome Zediloma s str 3.
Without a parietal band of shining nacre connecting both ends of the peristome Fractarmilla 4.
3 Shell globose, smooth, with fine growth lines, colour bluish black, often brownish on the earlier whorls, body whorl rounded Zediloma digna.
Shell, conical, or depressed conical, small, black or purplish black, often with yellow spots when young, body whorl angled at periphery (Fig. 3) Zediloma arida.
4 Shell spirally sulcate, dull or shiny black, or greenish black, frequently eroded on spire, may be spotted with white or yellow on the body whorl Zediloma (Fractarmilla) atrovirens.
Shell spirally ridged (ridges may be reduced by erosion) when small, purple, becoming more yellow with increasing size, with or without various patterns of purple, inner lip margined wholly with black or yellow or any combination of these two Zediloma (Fractarmilla) subrostrata.
Shell as above, but body whorl marked with 12–15 alternating bold transverse black and yellow or white stripes Zediloma (F.) subrostrata zebrina.

Family Trochidae
Subfamily Trochinae
Genus Melagraphia Gray, 1847

Type by monotypy Trochus aethiops Gm.

Animal having three pairs of epipodial tentacles, 8 claviform organs, and a double row of short blunt papillae between the epipodial tentacles. The margin of the inhalant siphon is divided into up to 30 short tentacles. (See Fig. 4.) Jaws present. Radula formula ∞ .5.1.5. ∞; central tooth with hooked lateral basal processes; lateral teeth with a lateral process which overlaps onto the adjacent tooth.

Shell solid, globose, depressed, imperforate, spirally sulcate, periostracum lamellose; columella white, sub-straightened, obtusely bidentate near base, spreading onto the outer lip; a nacreous band extends across the parietal region and unites both ends of the peristome.

Melagraphia aethiops (Gmelin. 1791)

1791.Turbo aethiops Gmelin, Systema Naturae, edition xiii, p. 3596, Type Loc. New Zealand (?) Type ?

1828.Trochus reticularis Gray, Wood, Ind. Test. Suppl., pl. 5, fig. 36.

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Figs. 4 and 5. Lateral view of Melagraphia aethiops showing epipodial papillae (p.). 5—Lateral view of Zediloma arida. Shell and operculum removed in both cases.

1834. Trochus Zelandicus Quoy and Gaimard, Voyage de l'Astrolabe, Zool. III, p. 237, pl.64, figs. 10 and 11.

1835 Monodonta reticularis (Gray), Gray in Yate's Account of New Zealand, p. 308.

1847. Melagraphia aethiops (Gmelin), Gray, Proc. Zool. Soc. Lond., XV, p. 145.

1853. Labio reticularis (Wood), A. Adams, Proc.Zool. Soc. Lond, XIX., p. 178, (not of Wood).

1853. Labio zealandica (Quoy and Gaimard) A. Adams, Proc. Zool. Soc. Lond., XIX, p. 178.

1857. Labio Zelandicus (Q. & G.) Gray, Guide to the Mollusca in the British Museum, p. 155.

1863. Monodonta (Diloma) aethiops (Gmelin), Troschel, Das Gebiss der Schnecken, II, p. 231, t. 23, fig. 5 (radula).

1873. Labio reticularis (Quoy), Hutton, Cat.Mar.Moll. N. Z. p. 37 (not of Quoy).

1873. Trochus (Diloma) aethiops (Gmelin), von Martens, Crit.List of Moll. N. Z. p. 32.

1880. Diloma aethiops (Gmelin), Hutton, Man. N.Z.Moll., p. 95.

1880. Diloma (?) concolour, (A. Adams), Hutton, Man. N.Z Moll. p. 96. Listed as a synonym by Suter (1913) but fide Iredale this name applies to Trochus lineatus Da Costa.

1883. Diloma aethiops (Gmelin), Hutton, Trans.N.Z. Inst. 15, p. 125, pl. 15 fig. A. (radula).

1884.Monodonta (Diloma) aethiops (Gmelin after Chemnitz), Hutton, Proc.Linn. Soc. N.S.W. p. 365.

1885. Neodiloma aethiops (Gmelin), P. Fischer. Man. de Conch. p. 820, pl. 50, fig. 12.

1889.Monodonta aethiops (Gmelin) Pilsbry, Man of Conch Ser.1, XI, p. 98, pl. 19, figs. 99, 100, pl. 20, fig. 19.

1893.Monodonta aethiops (Gmelin), Hutton, McLeay Mem.Vol.Linn. Soc N.S.W., p. 71 (occurrence as a Pliocene fossil at Wanganui).

1897. Monodonta (Diloma) aethiops (Gmelin), Suter. Proc malacol. Soc Lond., 2, p. 262.

1913. Monodonta (Diloma) aethiops (Gmelin), Suter, Man N. Z. Moll, p. 116, pl. 39, fig. 3.

1927. Melagraphia aethiops (Gmelin), Finlay, Trans. N.Z. Inst., 57, p. 352.

1938. Diloma (Melagraphia) aethiops (Gmelin), Wenz, Handbuch Paläozool. Gastrop., 2, p. 301, fig. 660.

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Shell and Operculum

Shell, imperforate, globose, conical solid, thick.Sculpture, narrow spiral grooves, 11–13 on body whorl, the whole of the whorl being ornamented with numerous fine lamellose, collabral striae; ornament usually reduced on the earlier whorls by subsequent erosion. Colour, light brown when dry, dark grey when wet, often with a greenish tinge due to algal growths, weathered specimens purplish brown, often with white spots or patches where the periostracum has been removed. Periostracum lamellose. Spire, conic with apex somewhat rounded, varies to some extent with habitat; specimens from exposed shores often have depressed spires, those from very sheltered situations often with more acute spires, and consequently relatively taller.Protoconch mostly eroded, conical, 3–4 whorls, whitish with dark transverse stripes, whorls strongly spirally ridged; periostracum not lamellose. Suture, impressed, linear, somewhat buttressed below. Aperture, large, oblique, pearly within. Outer lip, regularly convex, sharp, margined within by a black band, often spotted with white. Black band backed by a band of opaque white. Columella, short, sub-straightened, white, obtusely bidentate near base. Base spreading into aperture and forming a thickened, white, shelf-like callosity in the abapical portion of the outer lip just below the broad white band. Basal callus pad broad, white, concave, bounded on topographic right by a broad chocolate coloured band. A nacreous band may connect both ends of the peristome across the parietal region. Posterior to the parietal band of nacre is an area of blue-black spotted with white. Operculum circular, thin, wholly corneous, nucleus central, sinistrally multispiral; attached to the dorsal surface of the posterior part of the foot by an anterior elliptical zone of attachment.

External Morphology

Head black with a bluish tinge, margined below by a yellow band; ventral surface of the snout white. Epidermis finely mammillated. Dorsally arise a pair of stout eye-stalks; these are black above and light green below, the outer lateral region being produced into a fin, which on the left side is tinged with green. On the right eye-stalk the lateral fin is continuous with the cephalic epipodium, which in this region forms the exhalant siphon. Dorsally and medially to the bases of the eye-stalks arise a pair of cephalic lappets. Eyes small, black, and circular, occupying about one-third of the subterminal dorsal area of the eye-stalks.The extensile and mobile cephalic tentacles are situated between, and slightly ventral to the cephalic lappets and eye-stalks.When extended they are long, slender, tapering structures.Dorsally the tentacles have a median longitudinal white stripe, flanked on either side by a brown stripe. The ventral surface of the tentacles is a light jade green colour. The whole surface of the tentacles is covered with numerous small, transparent, bluntly-ending papillae. Immediately posterior to the left eye-stalk is the left cephalic epipodium which in this region forms the inhalant siphon. The ventral margin of the inhalant siphon is divided into a number of small, slender tentacles up to 30 in number. The tentaculate region of the inhalant siphon has a marked blue sheen, but the upper portion is grey-white (lighter above than below), with a marked grey-brown stripe one-third of the width of the siphon. Immediately posterior to the eye-stalk is another similarly coloured but smaller stripe. The tentaculate margin to the epipodium ceases at the origin of the most anterior epipodial tentacle; from here posteriorly the edge of the epipodium may be thrown into folds, but is not digitate. In the region of the operculum the epipodium is red-brown in colour. There are normally three epipodial tentacles on each side; one at the posterior margin of the siphons, another usually smaller tentacle is found near the posterior margin of the foot, and the third one of intermediate size is found about half way between the first and the last. Hutton (1883) mentions that a fourth epipodial tentacle is sometimes found on the left side of this species. All epipodial tentacles arise from the side of the foot just below the epipodium, and are similar to the cephalic tentacles except that the transparent papillae are not so numerous. Near each epipodial tentacle is a small, cylindrical, blunt-ended structure called the claviform organ by Pelseneer (1906). There is a fourth claviform organ on each side not associated with an epipodial tentacle; this is the most anterior one and is to be found under the anterior third of the inhalant and exhalant siphons.The claviform organ associated with the first epipodial tentacle is situated immediately posteriorly to the tentacle, and not ventrally to it as is the case with the more posterior ones. Between the second, third, and fourth claviform organs is found a double row of fleshy papillae. These do not occur in any of the other New Zealand species mentioned here. The sides of the foot are finely mammillated, brown in colour, with a reticulate pattern of shining green. The ventro-lateral margins of the foot are marked with dots of black and white. The sole of the foot is white. The posterodorsal part of the foot bears the operculum, at the anterior margin of which is a small fleshy ridge. Posterior to the attachment of the operculum is a small whitish area marked with a varying number of black chevrons.

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The Mantle Cavity

The anterior part of the visceral hump is enveloped dorsally by the mantle, the anterior part of this is creamy-white, the ventral edge is a striking bright green, whilst the dorsal portion which comes into contact with the inner margin of the outer lip of the shell is patterned with black. The patterns of black correspond exactly with similar markings in the shell. Figure 6 shows the mantle cavity of M. aethiops as it appears when opened by an incision along the right side of the animal, with the mantle reflected over to the left. The dominant feature of the opened mantle cavity is the creamy-white gill which extends the full length of the mantle cavity and protrudes a little anteriorly. It is typical of the aspidobranch type, and is bipectinate throughout its length: the filaments of the left side are shorter than those of the right. This condition obtains throughout the length of the gill, not only in that portion which is enclosed by the afferent and efferent membranes.

The osphradium is a small, brown, button-like mound of tissue lying in the inhalant stream. On the roof of the mantle cavity lies the rectum, which in the entire animal takes a somewhat S-shaped course across the roof, passing through the heart, round the periphery of the left kidney, and under the right kidney duct. It then continues round the margin of the right hypobranchial gland to discharge a short distance anteriorly into the exhalant stream. The hypobranchial glands are yellow in colour and have smooth surfaces. The right gland is much larger than the left. This is to be contrasted with the condition in European trochids where all reports have described the reverse condition, except in Margarites helicinus, which has only the right hypobranchial gland (Fretter, 1955). The significance of this and other aspects of the morphology are discussed elsewhere. Posterior to the left hypobranchial gland lies the mass of the left kidney. The large aperture of this kidney is easily seen just posterior to the most anterior point of contact between the kidney and the rectum. The aperture is large and bounded by tumid lips. To the right of this aperture is the opening of the right kidney duct, the duct itself is stout, and passes for a time parallel to, and in this view on top of the rectum. The kidney ducts show no sexual dimorphism.

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Figs. 6–10. Mantle roofs of 6. M. aethiops, 7.Z. digna, 8.Z. arida, 9.Z. (F.) subrostrata, 10—Z. (F.) atrovirens. Lettering for all figures: a. anus, g. gill, l.g. left hypobranchial gland l.k. left kidney, r.g. right hypobranchial gland, r.k. right kidney duct, v transverse pallial vessel.

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Figs. 11–15.Central and lateral teeth of radula of 11a. M. aethiops; 11b. a common variant of the central tooth, 12. Z. digna; 13. Z. arida; 14. Z. (F.) subrostrata; 15a. Z (F.) atrovirens; 15b. a common variant of the central tooth.

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The radula of M. aethiops was first figured and described by Troschel (1863), but whilst his drawings of the denticulations of the teeth are fairly satisfactory, the representations of the tooth bases are inadequate. Hutton (1883) being apparently ignorant of Troschel's work also figured the radula. His drawings of the tooth bases are more satisfactory.Because Hutton figured only a few of the lateral teeth and does not mention that the form of the central tooth is fairly variable, a new figure has been prepared. The description which follows is based in part on earlier descriptions, Hutton's slide, and new preparations. The dental formula is ∞ .5.1.5. ∞. The central tooth is about as long as it is broad, with well marked, often hooked, lateral basal processes. The cutting edge of the central tooth may be horizontal or may dip steeply to the left or right. The cutting edge is armed towards the sides with two groups of 5–9 denticles, the median untoothed portion of the cutting edge is longer than the denticulate portion. (The development of denticulations on the teeth is extremely variable, depending on the portion of the radula examined, the middle part giving best results as there are fewer broken teeth than at the distal end, and the teeth are usually well developed by comparison with those from the inner end of the radula sac.) The five lateral teeth vary progressively in the shape of the basal part of the tooth as shown in Fig. 11, but the cutting regions are all fairly similar. These teeth have rounded blades, the lateral margins of which are divided into 5–7 cusps. The hind edge of the tooth (not shown in Fig. 11) also bears a similar number of cusps, but these are frequently not so well developed as those on the front edge. The marginal teeth are present in great numbers (shown as infinity in the dental formula—here “infinity” has a value in excess of 280). The marginals decrease progressively in size towards the edge of the radula, are relatively simple, and have cutting blades which are denticulate throughout the whole of the outer edge. The inner margin appears to be entire.


This species is probably the most widely distributed trochid on the rocky shores of North, South, and Stewart Islands; it is also found on Chatham Island, Kapiti Island, Hen Island, Little Barrier Island, Great Barrier Island, Great King of the Three Kings Group, and Codfish Island in Foveaux Strait. The species is not recorded from the Subantarctic Islands of New Zealand. M. aethiops is intolerant of great exposure to surf, but may be present on exposed coasts where local sheltered spots occur. A fairly stable rocky substratum is a prime requirement of this species.The zone occupied varies considerably with local conditions, but the species may usually be found on rocks between mean tide level and extreme low water spring tides.

Genus Zediloma Finlay, 1927

Type by author's designation Zediloma digna Finlay.

Animal with 3 pairs of epipodial tentacles, 8 claviform organs and no papillae between the epipodial tentacles; margin of inhalant siphon divided into up to 22 short tentacles. (See Fig. 5.) Jaws present. Radula has formula ∞ .5.1.5. ∞ or (∞ + 3) .5.1.5. (3 + ∞); central tooth with well developed slightly hooked lateral basal processes; lateral teeth with a lateral process which overlaps onto the next tooth.

Shell solid, globose, depressed or depressed conical, imperforate, spirally sulcate, or spirally ridged; columella arcuate or substratghtened, smooth or slightly nodulous, spreading slightly onto the outer lip, with or without a band of nacre extending across the parietal wall and uniting both ends of the peristome.

Subgenus Zediloma Finlay, 1927
Type species, Zediloma digna Finlay

Animal as in the genus, but with the margin of the inhalant siphon divided into 14–19 short tentacles. Rectum straight, or nearly so; right hypobranchial gland much smaller than left Shell with light spiral sulcations, base somewhat excavated, a band of nacre extending across the parietal region and uniting both ends of the peristome; columella arcuate, smooth.

Zediloma (Zediloma) digna Finlay, 1927

1853 Labio nigerimma (Gmelin). A. Adams Proc. zool. Soc. Lond, XIX, p. 178, Habitat: New Zealand, Coll. Earl. (not of Gmelin).

1880 Diloma nigerrima (L.). Hutton, Man. N.Z Moll N.Z p. 96 (not of L. nor of Gmelin).

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1884. Monodonta nigerrima (Chemnitz) Hutton, Proc Linn. Soc., N.S.W., p. 366 (not of Chemnitz nor of Gmelin).

1889. Monodonta (Diloma) nigerrima? (Gmelin). Pilsbry, Man of Conch. Ser. I. XI, p. 97, pl. 23, figs. 77, 78, pl. 20, fig. 18. (not of Gmelin).

1897. Monodonta porcifera (Watson), Suter, Proc. malacol. Soc., Lond., 2, p. 264, text fig. (radula) (not of Watson nor of Adams).

1913 Monodonta (Diloma) nigerrima (Gmelin). Suter, Man. N. Z. Moll. p. 114, pl. 38, figs. 5, 5a. (not of Gmelin).

1927 Zediloma digna Finlay, Trans. N.Z. Inst, 57, p. 353, pl 18, figs. 24, 25. Type Loc. St. Clair Beach, Dunedin Type in Finlay Collection. Auckland Museum.

Shell and Operculum

Shell imperforate, depressed, globose, solid. Sculpture, numerous (18–24) close spiral striae, frequently obsolete, the number of striae not constant even on one specimen, marked changes in their number appearing after growth checks; spiral striae crossed by numerous, usually fine, collabral growth lines. Colour usually bluish black, but often reddish brown, scarlet individuals are found, but are rare. In some localities the spire is often pearly through the erosion of the periostracum. Periostracum fairly thick, shining. Spire low, cyrtoconoidal, apex obtuse. Protoconch consists of two whorls completely lacking in ornament, smooth and blue-black in colour. The next whorl is lightly marked with spiral striae and may be reddish. Whorls about five when adult, convex, with a depression below the suture, base rounded and concave, eroded and iridescent posterior to the parietal band of nacre. Suture linear, strongly buttressed below by a well marked cord. Aperture oblique, large, with a greenish iridescence, marked with a number of longitudinal lines passing into the throat. Outer lip convex, rather thin, sharp, margined within by a very narrow black band, followed by a wider band less brilliantly iridescent than the rest of the aperture. Columella concave, arcuate, lacking teeth, broad, white, excavated on the face, spreading on to the inner surface of the outer lip, where it forms a short narrow shelf; the white material of the columella extends into the aperture along the inner surface of the suture line Parietal region crossed by a shining nacreous band which connects both ends of the peristome. Operculum as in Melagraphia aethiops.

Morphology of Soft Parts

The morphology of the head region is similar to that of M. aethiops, but differs in that the lateral fin of the right eye-stalk is fused with the anterior margin of the cephalic epipodium only through the basal third of its length. In all specimens examined the margin of the inhalant siphon was divided into 19 tentacles, instead of up to 30 in M. aethiops. There are no papillae between the epipodial tentacles; the transparent papillae on the epipodial tentacles are smaller and fewer in number than in M. aethiops. The arrangement of the claviform organs and the epipodial tentacles is somewhat variable in this species. It is not unusual to find animals with but a single pair of epipodial tentacles, and specimens have been found in which these are entirely lacking. When an epipodial tentacle is absent the claviform organ normally associated with it is usually, but not necessarily, absent. Similarly, even though the tentacle may be present the claviform organ may be absent. There is nothing to suggest that these abnormalities are the result of injury. The most anterior claviform organs (under the siphons) have been present in all specimens examined. Departures from the “normal” state (3 pairs of epipodial tentacles and 4 pairs of claviform organs) occur in about 30% of the Banks Peninsula material examined.

The head, snout, cephalic tentacles, and eye-stalks are black with a bluish sheen; the sides of the foot when contracted appear black, but when extended are grey. The dark coloured stripe on the upper part of the inhalant siphon of M. aethiops is here represented by a smaller, and more irregular pigmented patch.

Within the mantle cavity the rectum takes an almost straight course across the mantle roof (Fig. 7). Associated with the straightened rectum is a change in the relative size of the left and right hypobranchial glands. The right hypobranchial gland is much the smaller and narrower of the two, the left gland is long, and in the posterior region wide. The surfaces of both glands are smooth.

The radula described and figured by Hutton (1883) under the name Diloma nigerrima (Chemnitz) does not belong to this species but to Zediloma (Fractarmilla) atrovirens (Philippi). Hutton's description of the animal from which he presumably extracted the radula also fits Z. atrovirens Suter (1897) considered Hutton's figure to refer to Monodonta coracina (=

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Z. arida), but he was unaware of the nature of the radula of Z. (F.) atrovirens. Suter being aware of Hutton's error figured the radula of Z. digna in 1897. This figure was reproduced again in 1913, but in neither publication is there a description, and in view of the fact that the tooth bases are incomplete in his figure a new one has been prepared. Suter (1913) gives a reference to a figure of the radula of Monodonta nigerrima (Gmelin) in Troschel (1863). If he had compared this figure of the true South American form with his own figures, it is probable that he would have solved the long standing confusion of the New Zealand species with the conchologically very similar South American one.

Central tooth broader than high, symmetrical, with large lateral basal processes. The cutting edge of the central tooth is horizontal and armed at the mid point with a large broad cusp; this is flanked on either side by seven smaller cusps, of which the third from the centre is the largest, the remainder decreasing in size on either side of it. The five lateral teeth have bases as shown in Fig. 12. The cutting blades of these teeth are short and peg-like, with up to six small postero-lateral denticulations. The marginal teeth vary greatly in size. The three inner marginals are very much larger than the succeeding ones, the innermost one conspicuously so, having a cutting edge longer than the total height of the lateral teeth. The outer margin of the cutting edge is finely denticulate. There do not appear to be any denticulations on the lower, or inner edge of the marginals. The dental formula of Z. digna is best expressed as (∞ + 3) .5.1.5. (3 + ∞) in contrast to the formula ∞ .5 1.5 ∞ which applies to all the other species discussed here.


Zediloma digna usually lives in crevices between boulders between mean high water neap tides and extreme high water spring tides. It is often found browsing on cast up Durvillea, but not in situations exposed to direct sunlight. This species occurs on sheltered and exposed boulder beaches from Great King Island of the Three Kings group in the north to Campbell and Auckland Islands in the south, on Chatham Island, and on other islands just off the coast of the North and South Islands. Dr. R. K. Dell informs me that in the Auckland district it is rarely found, but when present may be as abundant as in the South Island where as many as 600 have been collected from a square metre of shore.

Zediloma (Zediloma) arida Finlay, 1927

1880. Trochus coracinus, Troschel. Fischer, Coquilles vivantes, p. 357, t. 110, fig. 6. (N.Z. Cook Straits) (not of Troschel).

1889. ? Monodonta coracina (Troschel). Pilsbry, Man. of Conch. Ser. 1, XI, p. 103, p. 19, fig. 94, pl. 31, fig. 28.

1897. Monodonta (Diloma) coracina (Troschel), Suter, Proc. malacol Soc., Lond., 2, p. 264, text fig. of radula (not of Troschel).

1913. Monodonta (Diloma) coracina (Troschel). Suter, Man. N.Z. Moll., p. 114, pl. 38, figure 4. (not of Troschel).

1923. Monodonta coracina (Troschel). Oliver, Trans. N.Z. Inst., 54, p. 526 et seq. (ecology) (not of Troschel).

1927. Zediloma arida Finlay, Trans N.Z Inst, 57, p. 353 (not = Trochus coracinus Troschel). Type Loc. Lyttelton Harbour. Type in Finlay Collection, Auckland Museum.

Shell and Operculum

Shell small, imperforate, orbicular-conoid, depressed, solid, lustreless when well grown, but may be shining when small. Sculpture in small specimens, a number of fine spiral striae crossed by numerous very fine collabral growth lines; in older specimens obsolescent growth lines are frequently all that is visible. Colour usually purplish-black in adults, rarely jet black, may be sparsely spotted on the body whorl with white; young specimens are shiny black with bright yellow spots. Periostracum fairly solid, but frequently eroded from the earliest whorls. Spire low and arched in young specimens, but conical with rounded apex in mature individuals. Protoconch 2½–3 whorls, light in colour with a number of fine spiral striae. Whorls 4–5 in adults, slightly convex, body whorl slightly convex above, obtusely angular at the periphery and almost flat below this. Suture linear, margined below by a stout buttress. Aperture oblique, with a pink and greenish iridescence. Outer lip sharp, margined with black, followed by a semi-opaque band. Columella white, arcuate, untoothed, callus-pad small, base excavated.

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Parietal region with a shining nacreous band connecting both ends of the peristome; posterior part of base eroded. In large specimens a bright green area is often found at the columella end of the peristome Operculum as in M. aethiops.

Morphology of Soft Parts

The animal of Zediloma arida is very similar externally to that of Z. digna, only differing from that species in the following respects. The tentaculate region of the inhalant siphon bears 14–19 tentacles which are relatively longer than in digna. The cephalic tentacles are dark blue with a median longitudinal black stripe on the dorsal side. All specimens examined have had 3 pairs of epipodial tentacles and 4 pairs of claviform organs (Fig. 3). The mantle edge is light green above, and black below A few specimens have been found in which the sole of the foot was a light apricot colour instead of the normal white.

Within the mantle cavity the chief distinguishing feature is the hypobranchial glands. In all specimens examined except one, the two glands were united across the rectum, and not separated by it as in the other species studied. The left hypobranchial gland is the larger and in the posterior portion lying nearest the gill it is somewhat folded (Fig. 8).

The Radula

Suter (1897) figured the radula of this species under the name Monodonta coracina (Troschel) The figure is incomplete, badly reproduced, and there is no description. A new figure and description are given here.

Central tooth almost as high as it is wide, with slightly hooked lateral processes; from the upper part of each lateral process arises a thin, weak supporting lamella; the cutting edge is arcuate, with a marked central eminence; the central part is entire, but the lateral regions bear 9–10 very fine, sharp denticles. The five lateral teeth have sharp cutting blades, armed on their inner edge with seven clearly distinguishable cusps; the outer edge has six. The tooth bases vary in shape as depicted in Figure 13. The marginal teeth next to the laterals are the largest, the series decreasing regularly in size towards the outer edge of the radula. The three very large inner marginals of Z. digna are not developed here. The cutting edges of the marginal teeth appear to be entire.


Zediloma arida is widely distributed throughout the coasts of the North, South and Stewart Island, and also on Auckland, Campbell and Chatham Islands. It is not of such general occurrence as the other rocky shore species, being apparently restricted to areas where the water is often turbid and where sheltered retreats are present. Usually found between mean tide level and high water neap tides.

Subgenus Fractarmilla Finlay 1927

Type species by author's designation, Labio corrosa A. Adams.

Animal as in the genus, but with the margin of inhalant siphon divided into up to 22 short tentacles. Course of rectum across pallial roof arcuate; left hypobranchial gland not more than 1½ times the area of the right gland. Shell globose or conical, spirally ridged or sulcate; columella substraightened, slightly nodulous; base not so excavated as in Zediloma s. str.; no continuous band of nacre extending across the parietal region and uniting both ends of the peristome.

The members of this subgenus have long proved troublesome to conchologists. The species referred by Finlay to this subgenus fall into two groups, the mud-flat species Zediloma (Fractarmilla) corrosa (A. Adams, 1853) and Z. (F.) subrostrata (Gray, 1835) and the rocky shore species Z. (F.) atrovirens (Philippi, 1851) and Z (F.) morio (Troschel, 1851). If one uses the descriptions of these species provided by Suter (1913) it is found that some of the material is readily referable to one or other of the species, but that much of it appears to fall between the two species in each group, and cannot with certainly be assigned to either. Since the mud-flat group contains the subgenotype it will be discussed first.

In his 1946 check-list Powell admits two subspecies of corrosa, plumbea (Hutton, 1883) and zebrina Powell, 1946a. The name plumbea was dropped without comment from the 1957 check-list. All of the mud-flat forms are closely related, and occupy the same type of habitat; tidal flats in sheltered harbours and inlets where the salinity does not regularly fall below about 15°/00 for any length of time.

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Accounts of the distribution of the two species are all fairly similar; Suter (1913) corrosa “on mud flats of the South Island; Heathcote Estuary; Dunedin”, and subrostrata “Northern part of the North Island as far south as Tauranga”. Finlay (1927) wrote “it (subrostrata) represents the Forsterian form (corrosa) in the Cookian province”, whilst Powell (1946 and 1957) gives the distribution of corrosa as Forsterian and Cookian, and subrostrata as Cookian and Aupourian. The distribution of the subspecies will be discussed later. As long as one is handling samples of a dozen or more adult specimens from localities in the northern part of the North Island or the southern part of the South Island little difficulty is experienced in making determinations on the basis of colour patterns mentioned in the existing descriptions. Determinations may not be made on the basis of Suter's key alone, as the key runs “Whorls spirally sulcate… corrosa. Whorls spirally ridged…… subrostrata”. I have never found a sulcate specimen of corrosa, and the juveniles of both species have well developed ridges whilst the adults of both are very variable in this respect. The subnodulose condition of the spiral ridges on the body whorl mentioned by Suter is an optical illusion resulting from alternating light and dark coloured bands crossing the spiral ridges at nearly right angles. From the earlier work of Hutton (1883) it should be possible to determine the species on radular characters as he notes “the eared central tooth of subrostrata differs from all other species of Diloma”. It seems that Hutton did not examine any number of radulae of any one species of the group, for if he had it is to be expected that he would have noticed the considerable amount of variation present even in this structure, and certainly that the majority of individuals of corrosa have cared central teeth. The original descriptions are very similar, brief, without figures, and the types have not been figured since. Type localities do not help: subrostrata “Eastern Coast, New Zealand”, and corrosa “New Zealand”.

When large collections from a number of localities are compared it is seen that the development of spiral ridges, the colour patterns developed on the body whorl, and the inner surface of the outer lip are very variable within each population, and to some extent between populations. Body whorl coloration is variable and is associated with age, the juveniles are invariably dark, and tend to become lighter as they grow larger and older. Populations from the far north include a number of individuals with plain yellow body whorls in the adult stages. Others from the same localities are marked with dark transverse wavy lines overlying the yellow ground of the body whorl. To the south the plain yellow body whorl types tend to disappear over the region from Napier to Banks Peninsula, only to turn up again on Otago Peninsula and Stewart Island. The transverse patterns of purple or purplish black or brown mentioned above tend to become coarser and darker in colour in the south (Napier, Porirua Harbour, Queen Charlotte Sound and Pelorus Sound). Specimens from near Point Howard, in Wellington Harbour, have very coarse coalescing patterns of dark purple or black. The populations from Banks Peninsula southwards are characterised by the great amount of erosion which has usually taken place on the adult shells, and the dark stripes frequently merge completely on parts of the body whorl to give patches of dark colour. Often in these populations the whorl becomes plain white or vellow after passing through a stage where the dark patches become reduced to spiral stripes of purplish black. None of these conditions is peculiar to any particular population, practically any condition can be found in any population, as well as intermediates between the conditions described above.

The markings on the inner surface of the outer lip vary in a similar fashion, and to some extent with the body whorl coloration. Generally the lighter the appearance of the body whorl the smaller is the amount of black within the lip, and vice versa. Since lip coloration roughly follows the body whorl coloration and as mentioned earlier all juveniles are dark in colour, it follows that the dark markings.

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within the lips of juvenile shells are large and bold. As the shell becomes larger, and usually lighter in body whorl coloration, the lip markings of black generally become smaller. In the Auckland specimens with a wholly yellow body whorl the lip may be quite without dark markings, or these may be present as a row of dots. In shells with transverse zigzag stripes on the body whorl the markings may be present as a narrow band of black, or a series of broad, bold patches. In populations from more southern localities there is a general tendency for there to be more black on the lip. There is no strict progression from north to south. (Specimens from Pelorus Sound resemble those from Cheltenham Beach, Auckland, more closely than ones from Napier, Porirua Harbour, or Petone). Some of the variations which are to be found in lip markings are shown in Fig. 16. Every intermediate condition between these selected examples is to be found.

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Fig. 16. Z. (F.) subrostrata, drawings showing the range of variation found in the colour patterns on the inner surface of the outer lip.

In association with the gradual coalescing of the undulating transverse bands of colour on the body whorl the coloured region in the aperture of the shell changes from a pattern of alternating light and dark transverse bands to a solid strip of blue-black passing into the aperture. Neither of these conditions is restricted to any part of the geographical range of the species.

The juveniles, or small shells up to 10 mm in diameter from all localities examined, are strikingly uniform, and whilst it is possible with experience to assign collections of adults to probable populations this is quite impossible with juveniles Juveniles all possess well marked spiral ridges and are dark in colour. The inner surface of the outer lip of most small specimens is lightly marked with small dots or bars of yellow or orange. From Napier southwards individuals are found with the lip wholly black. These represent the form called Diloma plumbea by Hutton. Individuals of plumbea-type are found in all the populations in the South Island and the southern part of the North Island. Some juveniles from Cheltenham, Auckland, come very close to this type, but usually retain some spots of light colour in the outer band of the lip. As specimens agreeing with the types of this species are small, are always found in association with other individuals of subrostrata or corrosa type, and specimens which have recently “grown out of” the plumbea stage are relatively not uncommon, it is considered that this subspecies should be dismissed. (The co-types of this subspecies consist of two specimens of corrosa and one very eroded specimen of Melagraphia aethiops. This latter specimen is interesting and may explain why Hutton commented that “the central tooth of plumbea is remarkably like that of M. aethiops”, and also why his description of the animal fits M. aethiops better than Z. corrosa.) Z. (F.) corrosa plumbea is then a juvenile variant found at varying but usually low (2–5%) frequencies in at least the Cookian and Forsterian populations of mud-flat species.

In view of the adult variation the problem arises, does the uniformity of the juveniles indicate close affinity between the forms (juveniles of closely related species often being more alike than the adults), or does this indicate the existence of a single species? In other words, do the subrostrata and corrosa populations really represent two distinct species or subspecies? Earlier workers have recognised at least two species, but most opinions appear to have been based on small samples, and the systematists who have maintained this position have never mentioned the

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populations from shores in the vicinity of Cook Strait. Present day conchologists hesitate to assign these to either corrosa or subrostrata. Careful examination of material from Auckland, Porirua, Canterbury, and Otago has not yielded one difference in external morphology, the radula, or the morphology of the mantle cavity which would enable separation of even the two ends of the range of variation. In the shells, if the general erosion of the southern forms be ignored, we are left with colour patterns only to subdivide the mud-flat group into species or subspecies. Since there is great overlap in the colour patterns between different populations separation at the specific level cannot be defended. The problem remains, however, how much reliance can be placed on colour alone for separation at even the subspecific level. It has frequently proved unreliable in land and fresh water gastropods, and Purpura lapillus (Moore. 1936) is an example of its fallibility in the pectinibranchs. It is also to be remembered that these animals live only on a special type of shore, and each population is to some extent geographically isolated from the others. Environmental factors are known to alter the form and colour of some mollusc shells. Thus, even though the extent of geno-typic variation must for the moment remain unknown, it is possible that the different environments in which the populations live may have some effect in producing phenotypic variation. If separation of subspecies appears desirable, there remains the problem of dividing what has every appearance of being a close approximation to a continuum, and this in the face of the great similarity of juveniles which have grown quickly, and may have been influenced by the environment to a lesser extent.

In view of this I propose for the following reasons to merge the species known up to the present as Z. (F.) corrosa, Z. (F.) corrosa plumbea and Z. (F.) subrostrata under the name Z. (F.) subrostrata (Gray):–

  • 1. No differences can be found in the external morphology or the mantle cavity of animals referred to either of these species.

  • 2. The radulae of the forms known as corrosa and subrostrata are indistinguishable.

  • 3. No reliable differences can be found in the form of the shells.

  • 4 The great erosion of the shell of southern forms is probably the result of an environmental factor.

  • 5. The colour patterns on the inner surface of the outer lip and the body whorl vary considerably with the age of a specimen, and within and between populations to such an extent that separation on this basis would serve no useful purpose, if indeed it were possible.

In the early stages of this work an attempt was made to discover which forms of the mud-flat species had originally been called corrosa and subrostrata by the authors of these species. The description of corrosa given by Hutton (1880) is not a copy of the original, but a translation from the original Latin, and it contains an error; “cinereo lutescens” is translated as “dirty blue” instead of “ashy yellow”. At my request the late Mr. G. L. Wilkins, of the British Museum (Nat. Hist.) has examined the types of both species and supplied me with photographs of them. He reported on the types as follows “as far as I can see it (corrosa) is simply an eroded form of subrostrata, almost as though treated with acid, a little of the pattern being left on the spire. The body of the shell is rough and yellowish. The peristome appears to be the same as in subrostrata”. The syntypes of subrostrata are typical northern forms, and whilst the syntypes of corrosa are badly eroded, the lips of two specimens appear to be quite without dark markings in the lip, and on the third the marks are present as a row of black dots. As mentioned previously, the absence of dark markings in the lip is characteristic of populations from the Auckland district. On these grounds and from the study of Smith's (1874) figure of a Dieffenbach specimen of subrostrata determined by Gray (1843) I am.

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convinced that Labio corrosa A. Ad. is synonymous with Monodonta subrostrata Gray.

Zediloma (Fractarmilla) subrostrata (Gray)

1835. Monodonta subrostrata Gray, in appendix to Yate's account of N. Z. p. 308. Types: four syntypes in Cuming Collection Brit. Mus. (N.H.). Type Loc. Eastern Coast, New Zealand.

1853. Labio subrostrata (Gray). A. Adams, Proc. zool. Soc. London, XIX, p. 178.

1853. Labio rudis A. Adams, Proc zool. Soc. Lond, XIX, p. 179 (preocc. by Monodonta rudis Gray, 1826 fide Iredale, 1915).

1853. Labio corrosa A. Adams, Proc. zool. Soc. Lond., XIX, p. 18. Types: three syntypes in Brit. Mus. (N.H.) Type Loc. New Zealand.

1853. Chlorostoma undulosum A. Adams, Proc. zool. Soc. Lond., XIX, p. 182. Types: three syntypes in Cuming Collection, Brit. Mus. (N.H.). Type Loc. New Zealand.

1854. Trochus attritus Hombron et Jaquinot, Voy. Pole Sud., Zool., 5, p. 57, Pl. 14, Figs. 19–20. Types: holotype and four paratypes in Mus. Nat. D'Hist Nat, Paris. Type Loc. New Zealand.

1858. Diloma undulosa (A. Adams) H. & A. Adams, Genera Recent Moll. 1. p. 240.

1858. Trochocochlea corrosa (A. Adams). H. & A. Adams, Genera Recent Moll, 1, p. 420.

1858 Trochocochlea subrostrata (Gray) H & A Adams, Genera Recent Moll, 1. p. 420.

1873. Trochus (Labio) corrosus (A. Adams). Martens, Crit. List Moll. N.Z., p. 32.

1873. Trochus (Diloma) subrostratus (Gray). Martens, Crit. List Moll N. Z., p. 33.

1873. Labio Hectori Hutton, Cat. Mar Moll. N.Z., p. 37. Types: two syntypes M. 199, in Dominion Museum, Wellington. Type Loc. West Coast, South Island, N. Z.

1874. Trochus (Labio) subrostratus (Gray). Smith, Voy. Erebus & Terror, Moll., p. 14, t. 1, Fig. 14.

1874. Trochus (Chlorostoma) undulosus (A. Adams). Smith, Voy. Erebus & Terror, Moll., p. 14, t. 1, Fig. 15. (Fig. of syntype).

1880. Diloma hectori (Hutton). Hutton, Man. N.Z. Moll., p. 95.

1880. Diloma undulosa (A. Adams). Hutton, Man. N.Z. Moll., p. 96.

1880. Diloma corrosa (A. Adams) Hutton, Man. N.Z. Moll., p. 96.

1880. Trochocochlea subrostrata (Gray). Hutton, Man. N.Z. Moll. p. 96.

1880. Monodonta attrita (Hombr. et Jacq.). Fischer, Coquilles Vivantes… p. 186, t. 61, Fig. 2.

1882. Diloma undulosa (A. Adams). Hutton, Trans. N. Z. Inst. XV, p. 125, Pl. 15, Fig. C. (radula).

1882. Diloma corrosa (A. Adams) = (D. hectori Hutton). Hutton, Trans. N.Z Inst., XV, p. 126, Pl 15, Fig. D (radula).

1882. Diloma plumbea Hutton, Trans. N.Z. Inst., XV, p. 126, Pl. 15, Fig. E (radula). Types: two co-types in Canterbury Museum, Christchurch. Type Loc: Heathcote Estuary, Christchurch.

1882. Diloma (?) subrostrata (Gray). Hutton. Trans N. Z. Inst., XV, p. 126, Pl. 15, Fig. G. (radula).

1884. Monodonta melaloma, Menke. Hutton, Proc. Linn. Soc. N. S.W., IX, p. 366 (not of Menke).

1884. Monodonta melaloma var. undulosa (Adams). Hutton, Proc. Linn. N. S. W, IX, p. 366.

1884. Monodonta melaloma var. plumbea (Hutton). Hutton, Proc. Linn. Soc. N.S.W., IX, p. 366.

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1884. Monodonta subrostrata Gray, Hutton, Proc. Linn. Soc. N.S.W., IX, p. 367.

1889. Monodonta subrostrata Gray. Pilsbry, Man. of Conch., ser. 1, XI, p. 101, Pl. 19, Figs. 89, 90.

1889. Monodonta melanoloma Menke. Pilsbry, Man. of Conch., ser. 1, XI, p. 104, Pl. 22, Figs. 56, 57.

1889. Monodonta hectori (Hutton). Pilsbry, Man. of Conch., ser. 1, XI, p. 484.

1889. Monodonta melanoloma var. undulosa (A. Adams). Pilsbry, Man. of Conch., ser. 1, XI, p. 105, Pl. 22, Figs. 45–47.

1889. Monodonta melanoloma var. plumbea (Hutton), Pilsbry, Man. of Conch., ser 1, XI, p. 105, Pl. 20, Figs. 15–17.

1893. Monodonta melanoloma Menke. Hutton, McLeay Mem. Vol. Proc Linn. Soc. N.S.W., p. 71 (occurrence as a Pliocene fossil at Wanganui). (Not of Menke.)

1897. Monodonta (Diloma) subrostrata Gray. Suter. Proc. malacol. Soc. Lond., 2, p. 266.

1897. Monodonta (Diloma) melanoloma Menke. Suter, Proc. malacol. Soc. Lond., 2, p. 266 (Not of Menke.)

1897. Monodonta (Diloma) melanoloma var. undulosa (A. Adams). Suter, Proc. malacol. Soc. Lond., 2, p. 267.

1897. Monodonta (Diloma) melanoloma var. plumbea (Hutton). Suter, Proc. malacol. Soc. Lond., 2, p. 267.

1913. Monodonta (Diloma) corrosa (A. Adams). Suter. Man. N. Z. Moll., p. 117, Pl. 38, Fig. 7.

1913. Monodonta (Diloma) corrosa plumbea (Hutton). Suter, Man. N. Z. Moll., p. 118.

1913. Monodonta (Diloma) corrosa undulosa (A. Adams). Suter, Man. N. Z. Moll., p. 118.

1913. Monodonta (Diloma) subrostrata Gray. Suter, Man. N. Z. Moll., p. 121, Pl. 38, Fig. 11.

1923. Monodonta corrosa (A. Adams). Oliver, Trans. N. Z. Inst, 54, p. 509 et seq. (erosion of shells, ecology).

1923. Monodonta subrostrata Gray. Oliver, Trans. N. Z. Inst., 54, p. 543 (ecology).

1927. Zediloma (Fractarmilla) corrosa (A. Adams). Finlay, Trans. N. Z. Inst. 57, p. 352.

1927. Zediloma (Fractarmilla) subrostrata (Gray). Finlay, Trans. N.Z. Inst., 57, p. 352.

1938. Diloma (Fractarmilla) corrosa (A. Adams). Wenz, Handbuch, Paläozool Gastrop., 2, p. 301.

1946. Zediloma (Fractarmilla) corrosa plumbea (Hutton) Powell, Rec Auckland (N.Z.) Inst., 3, p. 137.

1950. Zediloma (Fractarmilla) corrosa zebrina? Powell Fleming, N.Z. J. Sci. Tech. 31, (5) p. 31. (Not of Powell)

1953 Zediloma aff. corrosa (A Adams) Fleming. N. Z. Geol Surv Bull, 53, p. 273. (As fossil.)

1953 Zediloma (Fractarmilla) c.f. subrostrata (Gray). Fleming, N.Z. Geol Surv. Bull., 53, p. 245 (As fossil)

1953. Zediloma (Fractarmilla) corrosa (A. Adams). Fleming, N.Z. Geol. Surv. Bull., 53, p. 150. (As a fossil.)

Shell and Operculum

Shell globose-conical, solid, imperforate, surface dull, spire may be eroded when it appears dull white, or occasionally nacreous. Sculpture, 5–7 spiral ridges on upper part of the body whorl, usually well developed in small specimens but becoming obsolescent or absent in large specimens 7–8 finer closely spaced spiral ridges on the base. These are often reduced through erosion Colour is very variable. Small specimens up to 1 cm in diameter purplish-black,

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with indistinct yellowish or orange patterns appearing near the aperture, or may be clearly marked with alternating transverse zig-zag bands of brownish-black and yellow. With increasing size the yellow (or white) areas may increase in size at the expense of the dark stripes until in many cases a wholly yellow body whorl is produced. In others the purplish bands may coalesce so that the yellow appears only as spots. Spiral stripes of purple are found in some. All intermediates between these conditions are found. In a few specimens the stripes are pink and not purple Spire conical, acute. Protoconch, small, conical-acute, smooth, mostly eroded. Whorls, 5 when adult, convex, rapidly increasing, base convex. Suture well impressed. Aperture oblique, silvery. Outer lip convex, coloration very variable; in small specimens may be edged wholly with black, or the black may be interrupted by varying amounts of yellow (rarely orange), or with alternating stripes of black and yellow. In specimens of moderate size the lip is usually margined by a more or less continuous yellow or white band backed by a continuous black band In fully adult specimens the black band may be reduced to a series of irregular dots or be entirely absent. (See Fig. 16.) In all cases the black band, or the position usually occupied by it, is backed by a band of opaque white, equal in width to the two other coloured bands combined. Internal to the coloured bands the aperture is nacreous. Columella oblique, opaque white, nearly straight, broad and smooth with a slight callosity below. Basal callus pad opaque white, extending to half the width of the base, and passing in a narrow strip into the aperture of the shell Extending from the posterior end of the peristome into the aperture is another pearly strip. Between this and the strip arising from the callus pad is a median tract of transparent nacre, through which the colour pattern on the base of the preceding whorl is frequently visible. Operculum circular, thin, wholly corneous, formed of numerous gradually increasing whorls, nucleus central.

External Morphology

Zediloma (F.) subrostrata differs from M. aethiops in the following morphological features: There are 16–22 tentacles on the digitate margin of the inhalant siphon; the fin of the right eye-stalk is fused to the cephalic epipodium of that side through the basal quarter of its length; there are no papillae between the epipodial tentacles. Within the mantle cavity (Fig. 9) the only notable difference is in the left hypobranchial gland, which is relatively smaller than in M. aethiops and has its greatest length in the antero-posterior direction, which is to be contrasted again with M. aethiops, where the greatest length of this gland lies across the mantle cavity.

The coloration varies somewhat between individuals but is generally as follows: snout and sides of foot, yellow or greenish yellow with a reticulate pattern of black markings. The ventro-lateral margin of the foot is white, as is the sole of the foot. Cephalic lappets, blue-black with a broad margin of yellow; cephalic tentacles whitish with fine black reticulate markings, a black stripe occupying the median dorsal surface. The mantle edge is cream with variable black markings on the anterior dorsal surface which mirror similar dark markings within the shell. The gill lamellae differ from all other species studied here in having a number of small black pigment spots on the afferent margin. This gives the ctemdium a dirty appearance. The osphradium is bright yellow, which is to be contrasted with M. aethiops where the osphradium is brown.

The Radula

The central tooth is usually wider than it is high (Fig. 14), but occasionally specimens are found where the converse applies. The central tooth differs from all other species studied in the development of “ears” which are lateral expansions of the ascending processes arising from the slightly hooked lateral basal processes. The central part of the cutting edge is entire; the lateral regions each bear five fine denticles. The five lateral teeth have bases which are much truncated by comparison with the other species of the genus, and which vary in shape as in Figure 15 The cutting blades of the lateral teeth bear up to five cusps on either side. The marginals show the normal decrease in size from the inner to the outer portions of the radula. The anterior margin of the cutting edge of these teeth bears very fine denticles. The posterior margin appears entire.


North Island, South Island, and Stewart Island. In the course of the preparation of this paper material has been examined from the following localities. Dominion Museum Collection: Patterson Inlet, Tauranga Harbour, Kawhia Harbour, Port Ahuriri, Farewell Spit. N.Z. Geological Survey Collection. Anakiwa (Queen Charlotte Sound), The Grove (Queen Charlotte Sound), Kawhia Harbour, Kaitangata, Pt. Resolution (Auckland), Raglan Harbour, Bluff Harbour, Vaila Voe, Stewart Island, etc., Charteris Bay (Lyttelton Harbour), Hoopers Inlet (Otago Peninsula).

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Canterbury Museum Collection: Hokianga, Whaingaroa, Dunedin, Heathcote Estuary, Purau (Lyttelton Harbour). Personal Collection: Cheltenham Beach (Auckland), Napier, Porirua Harbour, Pt. Howard (Wellington Harbour), Shakespeare Bay (Picton), Curious Cove (Queen Charlotte Sound), The Grove Arm (Queen Charlotte Sound), The Head of Pelorus Sound, Nelson Haven, near Kaiteriteri, Wainui (near Takaka), Puponga, Ligars Bay, Heathcote Estuary, Governor's Bay, Head of the Bay, and Purau, in Lyttelton Harbour, Akaroa Harbour, Company Bay, “Shelly Bay” (near Portobello Marine Biological Station), Burns Point, Quarantine Island, Pulling Point, sand flats opposite Port Chalmers (Otago Harbour), Papanui Inlet, Hoopers Inlet, Riverton. My personal collection has been deposited in the Dominion Museum.

Zediloma (Fractarmilla) subrostrata zebrina Powell

1946. Zediloma (Fractarmilla) corrosa zebrina Powell, Rec. Auck. (N.Z.) Inst. 3, p. 137, Pl.–, Fig.–. Types: Holotype and paratypes in Auckland Museum. Type Loc. Breaksea Sound, New Zealand.

The status of this subspecies is problematical. Through the kindness of Dr. Powell I have been able to examine seven of the paratypes. These shells differ markedly in coloration from subrostrata from other parts of New Zealand. The animal and radula are unknown. When the subspecies was described Powell noted that it “is very uniform in the type locality”. I have examined material from three of the four “New Golden Hind” stations in Fiordland, where a somewhat similar form was collected. Fleming (1950) referred this material to zebrina. It does not agree with Powell's description, nor with the paratypes examined; there being 25–33 transverse stripes on the body whorls of the “New Golden Hind” material, but only 12–15 in the types. Whilst not zebrina, this material, which is poorly preserved, also differs slightly from material from other localities outside Fiordland. The differences mainly concern the prominence of the stripes on the body whorl. This raises the problem of whether or not the southern fiords support two distinct forms of subrostrata. In this connection it is interesting to note that Resolution Island forms part of the southern shore of Breaksea Sound, the type locality of zebrina, and that material from Goose Cove, on the south shore of Resolution Island, is not zebrina. Large collections from as many Fiordland localities as possible are needed before the status of this subspecies can be decided. The original description covers the form adequately.

Zediloma (F.) Atrovirens and Z. (F.) Morio

The two species, Z. (F.) atrovirens (Philippi, 1851) and Z (F.) morio (Troschel, 1851) have been a stumbling block to most New Zealand conchologists since 1897. They are very similar, and considerable variation exists in the shells of each species Because of this in most cases they can only be tentatively separated, and then only with considerable difficulty. When an attempt was made to determine material of one of these species from Scarborough, Christchurch, and Menzies Bay, Banks Peninsula, using the key and descriptions provided by Suter (1913) it was found that when young these animals are best referred to morio, but when they grow larger they are referable to both species, but mostly to atrovirens. All specimens collected from Island Bay appear to be referable to morio.

Series of radulae have been examined from specimens tentatively assigned on shell characters to both species, and although a slight amount of variation is present in each group it was found impossible to separate them on this structure. Similarly the hypobranchial glands which have proved a useful taxonomic feature in the other species studied show no differences between the forms assigned to morio ′and atrovirens. The species are morphologically indistinguishable. Considerable differences have been observed in the habitat, behaviour and zonation of all other species,

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but no differences have been found in the habitats or behaviour of specimens tentatively assigned to morio and atrovirens. Here it is useful to note that when Suter recorded Z. morio from New Zealand for the first time (1897) he noted that it was “very variable in form”, and when discussing atrovirens he stated “found mostly with the preceding species from which it is not always easily separated”. It seems from the above discussion that the present systematic arrangement is not entirely satisfactory. It is possible that the two names are synonymous. The two descriptions appeared in the same work, two pages apart, but written by different authors. The type locality of atrovirens is given as “Bay of Tasmania, New Zealand” (= Tasman Bay?), whilst Troschel's morio is noted as “habitat unknown”. The location of any types of these species, if such exist, is unknown to me. The only mention of morio that I have found in the literature between the description of the species in 1851 and Suter's redescription of it in his 1897 revision of the New Zealand Trochidae is a reprinting of the original description and figures by Pilsbry (1889). Suter's 1897 and 1913 descriptions of morio differ in several respects from that given by Troschel, and it appears that he may have made an error in determining his New Zealand material as this species. Troschel noted that his shell was “thin”, a feature not mentioned in either of Suter's descriptions.

Therefore, from the arguments given above, the virtual impossibility of accurate determination in the majority of cases, which arises from the similarity of form of the animal, the radula and the shell, and the similarity of habitat; the fact that both descriptions were published simultaneously by different authors, and that there is no knowledge of the country of origin of Z. morio, I propose that the use of this name for the New Zealand mollusc described under this name by Suter be discontinued, and all New Zealand specimens previously referred to this name be now referred to Zediloma (F.) atrovirens which is known to be of New Zealand origin.

Zediloma (Fractarmilla) atrovirens (Philippi)

1851 Trochus atrovirens Philippi, Conch Cabinet., p. 148. t. 24, Fig. 12. Type:? Type Loc Tasman Bay, New Zealand.

1880 Monodonta atrovirens (Philippi). Fischer, Coquilles Vivantes … p. 373, t. 14. Fig. 1.

1883. Diloma nigerrima (Chemnitz). Hutton, Trans. N.Z. Inst., XV, p. 125, Pl. XV, Fig. B. (radula). (Not of Chemnitz nor of Gmelin.)

1884. Monodonta melaloma var. guttata Hutton, Proc. Linn Soc N.S.W., IX, p. 367, Types:? Type Loc. Nelson.

1889. Monodonta melanoloma var. guttata Hutton. Pilsbry, Man. of Conch., ser. 1. XI, p. 105.

1889. Monodonta atrovirens (Philippi). Pilsbrv, Man of Conch. ser 1, XI, p. 110, Pl 22, Figs. 48–50.

1897. Monodonta (Diloma) morio, (Troschel) Suter, Proc. malacol. Soc. Lond., 2, p. 263. (Not of Troschel)

1897. Monodonta (Diloma) atrovirens (Philippi). Suter, Proc. malacol. Soc. Lond., 2, p. 263.

1913. Monodonta (Diloma) atrovirens (Philippi). Suter, Man. N.Z. Moll., p. 117, Pl. 38, Fig. 6.

1913. Monodonta (Diloma) morio, (Troschel). Suter, Man. N.Z. Moll., p. 120, Pl. 39, Fig. 4. (Not of Troschel.)

1927. Zediloma (Fractarmilla) atrovirens (Philippi). Finlay, Trans. N.Z. Inst., 57, p. 352.

1927. Zediloma (Fractarmilla) morio, (Troschel). Finlay, Trans. N.Z. Inst., 57, p. 352. (Not of Troschel.)

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Shell and Operculum

Shell semi-globose, cyrtoconoidal to conical, solid, thick, imperforate. Sculpture. 14–20 spiral sulci on body whorl, these are variously developed, in some they are so fine that they are counted only with difficulty, whilst in others they may be well marked; striae more closely spaced on basal part of the whorl. Whole of body whorl crossed by fine collabral growth lines. Colour, mostly black or blue black, some a greenish black, surface may be shining and polished, or dull and slightly roughened, very pale green specimens occur rarely. Shell of young specimens especially may be marked with a varying number of white or creamish spots, usually more abundant on the base. Periostracum thick, shining, or dull, usually intact on the body whorl but often eroded on the spire. (Loss of periostracum appears to be an environmental feature.) Spire low, conical, or depressed with a marked downward flexure of the penultimate whorl, Protoconch conic, small, of 2–2½ smooth whorls either reddish brown or green in colour. Whorls 5–6 when adult, rapidly increasing, convex or subangular below, may be rather depressed below the sutural cord; base varies with size, convex in small specimens, but in larger ones tends to be flatter or concave. Suture impressed, linear, margined below by a buttress or cord. Aperture large, very oblique, pearly with green lustre within, marked with longitudinal lines passing into the throat. Outer lip convex, sharp, margined within by a narrow black band, followed by a wider band of opaque nacre, rest of the interior with a greenish iridescence In older specimens the iridescence is obscured by an increasimg amount of white nacre. Columella white, oblique, nearly straight, nodulous near base, spreading into the inner surface of the outer lip. Basal callus pad varies in extent with age, becoming relatively larger in older specimens; marked with a distinct tongue-shaped pit which extends towards the left end of the peristome, spreading slightly onto the inner surface of the last whorl. Left and posterior part of the peristome bounded on the outside by a band of bright green or greenish black. Operculum as in M. aethiops.

Morphology of Soft Parts

Z. (F.) atrovirens differs from M. aethiops in the following features: There are only 17–19 tentacles on the digitate margin of the inhalant siphon, there are no papillae between the epipodial tentacles, and within the mantle cavity in the relative size and subdivision of the left hypobranchial gland. In this species the left hypobranchial gland is larger than the right, and is subdivided into anterior and posterior regions by the transverse pallial vein (see Fig. 10) The coloration of the animal is distinctive. The dorsal part of the head is dark brown with a blue sheen, and near its ventral margin the snout is crossed by stripes of yellow, bottle green or black, and white. Eye-stalks yellow-green with some dark pigment near the tips; cephalic tentacles, dark blue with a median dorsal dark brown stripe Sides of foot, upper part blackish with a fine reticulate pattern of shining green, the lower third of the sides of the foot is yellow with a reticulate pattern of black. The dark streak on the inhalant siphon is obsolescent by comparison with M. aethiops. The mantle edge is very light green, practically white, with a narrow black line near the anterior margin. The hypobranchial glands are orange.

The Radula

The radula of this species has been figured and described by Hutton (1883) under the name Diloma nigerrima (Chemnitz). Apart from the fact that he makes no mention of the variation in the form of some of the teeth his figure and description are fairly satisfactory For completeness sake new figures and descriptions are given here.

The central tooth is taller than it is wide, with slightly curved lateral basal processes, the cutting edge may dip steeply to the right as in Figure 15a or may be horizontal as in Figure 15b. The central part of the cutting edge is entire, but is flanked on either side by a group of small denticles. The denticles vary in number within the groups from 4–7 and also in size, frequently those on one side are larger than those on the other. The bases of the lateral teeth vary progressively in shape as shown in Fig. 15a. The cutting blades may be short or of moderate length, and bear 4 or 5 cusps on each edge, those of the outer edge being usually stronger than those of the inner edge. The marginals show the usual decrease in size towards the edge of the radula, and bear numerous denticulations on both cutting edges.


Found under boulders on rocky shores between mean tide level and mean high water neaps throughout North, South, Stewart, and Chatham Islands. Also on Kapiti, Hen, Little, and Great Barrier Islands.

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Literature Cited

Cotton, B. C. and Godfrey, F. K., 1939. A systematic list of the Gastropoda … of South Australia Malacological Society of South Australia. Publication No. 1.

Cox, L. R. 1955. Observations on gastropod descriptive terminology. Proc. malacol. Soc. Lond. 31, 190–201.

Finlay, H. J., 1927. A further commentary on New Zealand molluscan systematics. Trans. N.Z. Inst., 57, 320–485.

Fleming, C. A., 1950. The molluscan fauna of the fiords of Western Southland. N.Z. J. Sci. Tech., 31B, (5), 20–40.

Fretter, V., 1955. Some observations on Tricolia and Margarites. Proc. malacol. Soc. Lond., 31, 159–162.

Gray, J. E., 1843. Fauna of New Zealand. Chapter 10 of Vol. II of E. Dieffenbach's Travels in New Zealand. J. Murray, Lond.

Hutton, F. W., 1880. Manual of the New Zealand Mollusca. Colonial Museum and Geological Survey, Wellington.

—— 1883. Notes on some branchiate Gastropoda. Trans. N.Z. Inst., 15, 113–131.

Kuroda, T. and Habe, T., 1952. Check-list and bibliography of the Recent marine Mollusca of Japan Tokyo.

Moore, H. B., 1936. The biology of Purpura lapillus, I. Shell variation in relation to environment. J. Mar. biol. Ass. U.K, N.S. 21, 61–89.

Pelseneer, P., 1906. A Treatise on Zoology, edited by E. Ray Lankester, Part V. Mollusca, Black, London.

Pilsbry, H. A., 1889. Manual of Conchology 11, Academy of Natural Sciences, Philadelphia.

Powell, A. W. B., 1946a. New Species of New Zealand Mollusca from the South Island, Stewart Island and Chatham Islands. Rec. Auckland (N.Z.) Inst., 3, 137–144.

—— 1946b. Shellfish of New Zealand (Second Edition). Whitcombe and Tombs, Christchurch.

—— 1957. Shells of New Zealand (Third Edition). Whitcombe and Tombs, Christchurch.

Smith, E. A., 1874. The Zoology of the Voyage of H.M.S. Erebus and Terror, Mollusca p. 1–7.

Suter, H., 1897. Revision of the New Zealand Trochidae. Proc malacol. Soc. Lond, 2, 260–283.

—— 1913. Manual of the New Zealand Mollusca. Govt. Printer, Wellington.

Thiele, J., 1930. Handbuch der systematischen Wetchtierkunde, I. Gustav Fischer, Jena.

Troschel, F. H., 1856. Das Gebiss der Schnecken. Zur Bedrungdung einer naturlichen Classification untersucht. Berlin.

Wenz., W., 1938. Handbuch der Palaozoologie, 6 Gastropoda Berlin.

W. C. Clark, M. Sc., Entomology Division, D. S. I. R., P.O Box 223, Nelson.