Genus Diplodon, Spix (1827).
Shell elliptical, rounded, elongated or trapezoidal, with rather low beaks which are more or less distinctly radially sculptured, the ridges usually curved and approaching below, with a low or scarcely developed posterior ridge; surface slightly concentrically sculptured, sometimes broken into fine nodules or corrugations; epidermis dull, rayless; hinge with two compressed pseudocardinals in the right valve, and one slender lateral, and two compressed pseudocardinals in the left valve, one in front of the other, and two laterals; nacre bluish to white, dull, often blotched; beak-cavities shallow; dorsal scars numerous, forming a row in the beak-cavity parallel with the hinge line.
Animal with the marsupium occupying nearly the whole length of the inner branchiæ, a few ovules sometimes being found in the outer gills; branchiæ rather large, angular at base, inner much the larger, united their whole length to the abdominal sac; palpi scarcely projecting posteriorly; mantle very thin, thickened on the edges; branchial opening papillose, separated from the smooth anal opening by a strong bridge; supra-anal opening not closed below. (Simpson.)
Type: D. ellipticum, Spix.
Subgenus Hyridella, Swainson (1840).
Beaks rather low, sculpture consisting of curved generally nodulous ridges, which approach below, but usually have a smooth area of shell between them; surface sulcate or sometimes corrugated and nodulous; epidermis rayless; teeth rather delicate, compressed, often somewhat rudimentary.
Animal having the embryos occupying the inner gills for the most part, which are united for their entire length to the abdominal sac; outer gills pointed below in the middle; palpi triangular; branchial opening papillose; anal opening smooth, not separated from the supra-anal opening. (Simpson.)
Type: Unio australis, Lamarck.
Diplodon menziesi, Gray (1843).
Unio menziesi, Gray, in Dieffenbach's “New Zealand,” vol. ii., 1843, p. 257. Unio aucklandica, Gray, l.c., p. 257. Unio waikarense, Colenso, Tasman. Journ. Nat. Sci., vol. ii., 1845, p. 250.
I am following C. Simpson in considering Unio aucklandica as a synonym only. On examining a rather large series of specimens from over twenty localities I tried to uphold it at least as
a subspecies, but I had to give it up, as I had numerous specimens before me which could be either assigned to menziesi or to aucklandica. There is no doubt that when the extreme forms only are compared one would feel inclined to take them for distinct species, but so it is with many other species, as for instance with Helcioniscus tramosericus. However, it is convenient to refer to aucklandica as a form of menziesi which is but little winged, and having the dorsal and ventral margins subparallel.
Unio waikarense will be dealt with further on when describing the mussels from Lake Waikaremoana.
(1.) Lake Taupo (Stat. 9—From dredgings up to 100 ft.).—There are eight specimens, representing quite young to half-grown forms, only one being highly winged. All are distinctly radiately striate, and some of the youngest specimens show the typical beak-sculpture beautifully. It is represented by the accompanying diagram (fig. 1). One of the larger specimens is very distinctly sculptured with elongate nodules on the lower half down to the ventral margin in the region below the beaks. The interior and hinge are the same as in specimens of subspecies hochstetteri, to be described further on. The largest specimen measures—Length, 42 mm.; height, 28 mm.; diam., 13 mm.
(2.) Lake Taupo (Stat. 9F—From dredgings up to 100 ft.).—The eight specimens have the same appearance as those of the last station; all of them have the outline of aucklandica, are finely radiately striate, and one clean olive-coloured specimen also shows nodulous ornamentation. Five quite young specimens have the beaks already so much eroded that no trace of the beak-sculpture is left. A few specimens have a light ferrugineous coating. The largest specimen shows—Length, 43 mm.; height, 28 mm.; diam., 13 mm.
(3.) Lake Waikaremoana (Stat. 14—Dredged in 50 ft.).—Compared with the type of Unio waikarense, Colenso, said to have been obtained in this lake, the four specimens collected at this station are much smaller, very little winged posteriorly, the dorsal margin subparallel to the ventral, darker in colour, and more solid; they are not concentrically sulcated, but only striated, and the marks of rest are much less distinct. All of them are finely radiately striated, a character always to be found in menziesi. The pseudo cardinals are typical, the upper lateral tooth in the left valve is much lower than the other, and crenate
in all specimens. Inside bluish-white, pearly, blotched with yellowish, especially in the umbonal cavity.
(4.) Lake Waikaremoana (Stat. 37—Dredged in 15 ft.).—Four specimens of different size, showing all the characters of those from Stat. 14.
In some specimens in my collection, kindly sent to me by Mr. Elsdon Best, and collected in Lake Waikaremoana, the radiate sculpture is distinctly nodulous, sometimes V-shaped, thus approaching D. websteri, Simpson. Typical specimens of aucklandica from creeks near Auckland show the same sculpture to a most marked degree, and I consider D. websteri as a D. menziesi in which the nodulous sculpture is developed to the highest degree.
I have seen, thanks to Mr. E. Best's great kindness, a large number of Diplodon from Lake Waikaremoana, but not one approaching Colenso's type of waikarense, which is a large, thin, yellowish-olive-coloured shell, having more the appearance of an Anodonta. The Waikaremoana specimens are all much smaller, thicker, darker in colour, and less winged. Mr. A. Hamilton, Director of the Colonial Museum, to whom I spoke about it, and who has visited the locality, suggests that Colenso did not get his specimens from Lake Waikaremoana itself, but from some small lake or lagoon in the vicinity. This seems to be correct, as I obtained, again through the unremitting kindness of Mr. E. Best, a number of specimens from a lagoon near Ruatoki, Tuhoeland, and these are typical waikarense, Colenso; one of them showing the same outline and the same dimensions as the cotype in the Canterbury Museum.
An error in Colenso's diagnosis of Unio waikarense (Trans. N.Z. Inst., xiv., 169) wants correcting. He says, “Posterior slope keeled.” This, however, is not correct: the shell is not keeled. How misleading such an incorrect statement is may be guessed from Simpson's remark on the species in his “Synopsis of the Naiades,” p. 890 (footnote): “Suter thinks this is a variety of menziesi, but Colenso states that the posterior slope is keeled. If this is so it must be quite different from the species.” There is also an error in the quotation in Simpson's work, as he gives the year 1841 as the date of the publication of the Tasman. Journ. Nat. Sci. vol. ii., whereas it is 1845. Colenso says that he discovered the shell in 1841, but his description was published four years later.
I have compared typical specimens of waikarense with many specimens of menziesi from about twenty different localities, and I am unable to separate the two. In all essential characters the two agree, and there remains nothing but to make Colenso's species a synonym of D. menziesi, Gray.
Diplodon menziesi, Gray, subsp. hochstetteri, Dunker (1862).
Unio hochstetteri, Dunker, Malak. Blätter, vol. viii., 1862, p. 153.
The type was collected by Hochstetter in the Waikato River.
(1.) Lake Taupo (Stat. 46—From dredgings in 10–30 ft.).—Six specimens, blackish-brown, much corroded round the beaks, most of them more produced and rounded posteriorly than the type, but some have the posterior end distinctly truncated and more or less biangular. In correspondence with Mr. C. T. Simpson I expressed the opinion that hochstetteri is a pathological subspecies,* having seen the same form amongst specimens of rugatus, Hutton, from the Kopuaranga River,† and also amongst menziesi from the River Avon. The young shells are invariably typical D. menziesi, but on growing larger the deformity constituting hochstetteri becomes more and more apparent. Some specimens are more affected, others less, thus producing a strongly truncated biangular posterior margin, or it remains only flatly rounded. At the posterior end the periostracum is produced in thick, foliated layers, and the inner margin is considerably thickened by pearly substance, forming large rugosities, and very often pearls adhering to the shell are met with. Loose small pearls of irregular form are only exceptionally found. In my opinion the cause of this is most likely some parasitic creature, as is the case in most of the pearl-producing bivalves. The outer exposed layers round the beaks are smooth, light-brown, waxy. The concentric striation is rather coarse, the marks of rest distinct and elevated. The inside is but little iridescent, except along the ventral margin, outside the mantle impression, grey to light-brown, sometimes blotched with brown; there are more or less considerable rugosities beyond those at the posterior margin. The dorsal scars in the umbonal cavity are small and deep, and to the number of two to four. The anterior adductor impressions are irregular in shape, and much deeper than the posterior ones, which are oval and shallow. Right valve with two pseudocardinals, the upper anterior forming a small lamella, the posterior being strong, compressed, high, triangular, and slightly corrugated. The single lateral tooth is regularly slightly curved and somewhat crenate at its posterior end, which is abruptly descending. Left valve with two pseudocardinals arising from a common base, both blunt, the anterior tooth larger and but slightly crenate. The two laterals are also curved, the lower of them is strongly lamellated and denticulated at its posterior end, which slopes down very gradually.
[Footnote] * C. T. Simpson, “Synopsis of the Naiades,” p. 889 (footnote)
[Footnote] † Trans. N.Z. Inst., xxiv., 275.
(2.) Lake Rotoiti (Stat. 8—Obtained by dredging in 12 ft.).—Two specimens of medium size, strongly concentrically ridged, umbones much eroded, posterior end subtruncated, not yet distinctly biangular. They represent the intermediate stage between the young menziesi and the full-grown hochstetteri. Interior olive-bluish, pearly, a row of small dorsal scars in the umbonal cavity, parallel to the hinge line. Muscular scars strongly impressed. Right valve with two very unequal pseudo-cardinals, the anterior small, lamellar, the posterior compressed, with a broad posterior base, crenulated, trifid; the lateral tooth slightly curved, narrow, high, truncate and corrugate posteriorly. Left valve with the two pseudocardinals coalescent, separated only by a groove, the anterior strong, triangular, rugose. The two laterals with crenate edges, obliquely truncated behind. The dimensions of the two specimens are—Length, 50 mm.; height, 34 mm.; diam., 16 mm.; and, length, 41 mm.; height, 29 mm.; diam., 12.5 mm.
Diplodon menziesi, Gray, subsp. rugata, Hutton (1883).
Unio rugatus, Hutton, N.Z. Journ. Sci., vol. i., 1883, p. 478.
Lake Waikare (Stat. 41—From dredgings in all parts of the lake; most common on sand and on stony shore).—Six specimens, two of which are quite young. The latter are winged posteriorly, the beaks already eroded, and there are three to six nodulous ridges descending in front of the umbo, nearly parallel to the anterior margin; they are concentrically finely thread-striated; colour olive-brown; the inside is bluish-pearly, yellow under the beaks. The larger specimens are subventricose and thin, less winged, the dorsal margin nearly parallel to the length-axis; the beaks are corroded, the anterior margin rounded, sometimes slightly truncate, the posterior margin produced and the ventral margin broadly convex. The concentric striae are irregular, rugose posteriorly, fine and more regular at the anterior end; the marks of rest are fairly distinct. Most of the specimens are partly covered with a dark-brown ferrugineous coating. Interior bluish, yellowish, or purple pearly, sometimes strongly blotched with olive. Muscular scars shallow. Right valve with two compressed triangular pseudocardinals close together, the anterior tooth smaller, lamellar and smooth, the posterior stronger, broader, and somewhat rugose. The lateral tooth is slender, curved, rugose on the edge of the posterior part. Left valve with two elongated, rounded, rugose pseudocardinals which sometimes coalesce, when only a slight notch indicates the original two teeth. The two laterals are long, slender, sinuate, and distinctly pectinate at the posterior edges. There are always a few small rather deep dorsal scars in the umbonal cavity.
Diplodon menziesi, Gray, n. subsp. lucasi.
Lake Manapouri (Stat. 35—Dredging in 60 ft.).—Three specimens; one of them may be taken as adult. Shell (figs. 2 and 3) oblong-ovate, very much compressed, thin and fragile, inequilateral, beaks low, eroded; surface with close strongly pronounced rest-marks and between them a few lines of growth, all close together and foliated at the anterior end. In the adult specimen the middle part has distinct radiate nodulous sculpture, partly V-shaped, but no such ornamentation is to be found on the young specimens. The straight dorsal margin is subparallel to the ventral margin, which is slightly sinuate; the anterior margin is angularly rounded, the posterior obliquely truncated and slightly produced. Nearly the whole of the shell is covered with a thin ferrugineous coating; the epidemis is olive-green, waxy. The ligament is small, not much raised. In the right valve the two pseudocardinals are compressed, small; the upper anterior tooth is a small, smooth lamella, the lower tooth is more elevated, conoidal, and strongly crenate; the lateral tooth is almost straight, thin and rugose at its posterior portion. In the left valve there is a rather long compressed lower anterior rugose pseudocardinal, the upper tooth is quite rudimentary; the upper lateral tooth is a little higher and more rugose posteriorly than the other. Interior bluish-white, pearly, a little blotched with olive in the umbonal cavity, where there are rather large and deep dorsal scars. The adductor-muscle scars are shallow. The young specimens are slightly winged.
This subspecies is nearest to the typical aucklandica, but is distinguished from it by its exceptionally compressed form, the thinness of the shell, the strongly marked and close concentric lines, the more tapering posterior margin, and the feebly developed pseudocardinals. The radiate nodulous sculpture is found in many specimens of menziesi and its subspecies. Adult specimen—Length, 45 mm.; height, 24 mm.; diam., 8 mm.
Type in my collection.
I have much pleasure in naming the subspecies in honour of its discoverer, Mr. Keith Lucas.
Diplodon lessoni, Küster (1856).
Unio lessoni, Küster, Conch. Cat., 1856, p. 135, pl. xxxvi., fig. 4. Type from New South Wales.
Lake Wakatipu (Stat. 37—Dredged in 10 ft. to 30 ft.).—Nine specimens were obtained. Shell (fig. 4) oblong, obliquely truncated behind, compressed, inequilateral, beaks strongly eroded in all specimens, no trace of sculpture left; surface with distant flatly elevated rest-marks, which, together with the intervening space, are covered with very fine thread-like concentric lines; towards the base and posterior margin the growth-lines are more distinct and slightly foliated. There is no trace of radial sculpture. The epidermis is olive to dark-brown, dull. Interior bluish nacre, with yellowish patches under the beaks, where there are several deep small dorsal scars. Right valve: The pseudocardinals are compressed, the anterior upper tooth is small, grooved, the posterior tooth much larger and crenate; the lateral tooth is slightly arched and rugose posteriorly. Left valve: Anterior pseudo-cardinal compressed, tongue-shaped, slightly rugose, the posterior subtriangular, deeply denticulate; the laterals are of nearly equal height and crenate posteriorly.
The largest specimen was selected for the diagram and description, and its dimensions are—Length, 51 mm.; height, 30 mm.; diam., 14½ mm.
I submitted specimens to Dr. W. H. Dall, Hon. Curator of Mollusks, U.S. Nat. Museum, for examination, and he very kindly informed me that according to the material in the museum they are D. lessoni, Küster. I have not seen any Australian specimens, nor the description and figure published by Küster, but I do not hesitate to accept Dr. Dall's view, although some may think it hazardous to refer our shell to a New South Wales species. I have not seen this species from anywhere else in New Zealand.
Dr. Von Jhering once suggested that a number of measurements should be taken to ascertain the range of variability in species of the family Unionidœ. I have measured a great number of specimens from New Zealand some years ago, and may publish the results later on. I have done the same for a number of the Unionidœ collected by Messrs. Lucas and Hodgkin, and
the result is contained in the following table. [The ciphers i. to xi. in the table indicate—i. Length of shell. ii. Greatest height. iii. Index of height (length = 100, height in % of it). iv. Distance of greatest height from anterior end. v. Index of position of greatest height (length = 100, distance of greatest height in % of it). vi. Diameter. vii. Index of diameter (length = 100, diameter in % of it). viii. Length of hinge line. ix. Distance of beak from anterior end of hinge line. x. Umbonal index (length of hinge line = 100, distance of beak from anterior end of hinge line in % of it). xi. Cardinal index (length of shell = 100, length of hinge line in % of it).]
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Mean of the three above||41||24||58||23||55||11||27||27||7||25||64|
|Mean of the four above||59||33||56||30||50||18||30||39||10||25||64|
|Cotype of waikarense||79||48||61||40||51||25||31||47||10||21||59|
|Mean of the four above||55||38||66||29||54||19||34||40||9½||24||72|
|From Lake Rotorua, typical||61||44||72||32||52||24||39||42||10||24||69|
|Mean of the four above||44||27||60||22||50||14||30||29||8||26||64|
|From Lake Pearson, cotype||52||33||63||26||50||17||32||35||9||25||67|