![Thumbnail: [Volume 62, 1931-32 page 20]](/journals/rsnz/images/rsnz_62/thumbnails/rsnz_62_00_0066_0020_ac_02.gif)
On the Occurrence of Strebloceras in New Zealand.
[Issued separately, 23rd May. 1931.]
The Family Caecidae contains some of the most curious looking mollusca. Caecum itself is little like a gasteropod, while Meioceras, Watsonia, Parastrophia, etc., exhibit most bizarre tubular shapes. So far, only one member of the Family has been known from New Zealand, Caecum digitulum Hedley, 1904, of which C. suteri Odhner, 1924, is a synonym. This is a featureless species, consisting merely of a smooth short curved tube, plugged at one end, but I have now to report a much more interesting addition to the Family in New Zealand.
All Caecids begin with a discoidal embryo, which in every genus but one is shed on adolescence, the decollated tube being plugged with a more or less spiked septum. The single exception is the genus Strebloceras, which represents a permanent early stage in the development of the group. In it the embryonic planorbid stage is not shed during life, and the tube subsequently twists out of its plane, so that the adult shell resembles a miniature twisted Lituites. All the Caecid genera seem to have existed for a long time, for practically all are recorded from the Eocene, and one doubtful species of Watsonia from the Palaeocene; nevertheless, Strebloceras is probably the ancestral genus from which the others are more or less directly derived. There are but few species of Strebloceras described; two from the English Bartonian (Eocene), one from the French Bartonian, one from the Stampien of Etampes (Oligocene), and two Recent species. Thus the discovery of a new species of Strebloceras in the Awamoan horizon (Miocene) of New Zealand is of considerable interest.
As there is much disagreement as to the genotype, a review of the literature becomes necessary.
Cossmann (Ess. Pal. Comp., livr. 9, p. 156; 1912) discusses the genus and the three species known to him and remarks, “le génotype—indiqué par Fischer et Tryon—est manifestment inexact, attendu que C. subannulatum de Folin, n'a été décrit qu'en 1869, tandis que Carpenter avait déja créé ce Genre en 1858…. Dans ces conditions, j'ai institué comme néotype de Strebloceras l'autre coquille publiée par Deshayes (C. Edwardsi).” Certainly the statement of Fisher and Tryon needed correction, but Cossmann's own action was illegal, as edwardsi is not amongst the species originally included by Carpenter.
Strebloceras was first proposed in the Proc. Zool. Soc. (Lond.) for 1858, pt. 26, p. 440. No type was named, and only two Eocene new species described, one of which must therefore be taken as type. I cannot find that anyone has made a previous valid designation, so I here nominate Carpenter's “S. cornuoides (Brown) n. s.,” the first species, as genotype. This was described from the Upper Eocene of Barton and the Oligocene of Hampstead, while solutum, the second
![Thumbnail: [Volume 62, 1931-32 page 21]](/journals/rsnz/images/rsnz_62/thumbnails/rsnz_62_00_0067_0021_ac_02.gif)
species, was from the Oligocene of Hordwell. These two species are both listed, but no genotype named, by R. B. Newton in his Syst. List Brit. Oligocene and Eocene Moli., p. 216, 1891. The quotation of Brown in connection with cornuoides refers to his Illust. Rec. Conch. Gt. Brit., ed. 1; Pl. 1, Fig. 49; 1827, where he introduces the generic name Cornuoides for two species, major and minor, which were subsequently identified as the young of other Caecidae. In the remarks on S. cornuoides, Carpenter mentions that specimens of it were alluded to by Forbes and Hanley; they, however, merely say (Hist. Brit. Moll., vol. 3, p. 178; 1853) “The genus (Caecum) appears to have begun, so far as we yet know, during the Eocene period, since, according to Mr. Searles Wood, a species of it has been found at Hordwell by Mr. Edwards.”
Six years after Carpenter's introduction of the genus, Deshayes (Anim. s. vert., Bass. de Paris, vol. 2, p. 302; 1864) described two species from the Paris Basin, Caecum lituus and edwardsi, which have since been referred to Strebloceras, though Cossmann has stated (Ess. Pal. Comp., livr. 9, p. 156; 1912) that lituus is a Watsonia. Cossmann (Ann. Soc. R. Mal. Belgigue, vol. 23, fasc. 3, sp. 22–2; 1887) published one more Tertiary species, C. bezanconi, from the Bartonian of Mont-Saint-Martin. Since Cossmann makes no reference to Carpenter's two species, but notes that Deshayes sent Carpenter specimens, it seems to me highly probable that bezanconi Cossmann and edwardsi Desh. may be synonyms of solutum Carp. and cornuoides Carp., from the same horizons.
The description of two Recent species completes the genus as known up till now. De Folin discovered the first living shells, three specimens from 40 fathoms off the Honolulu reefs; these he described (Proc. Zool. Soc. Lond. for 1879, p. 807) as Strebloceras subannulatum. From the Bottle and Glass Rocks and Long Bay, Sydney Harbour, Hedley (Proc. Linn. Soc. N.S.W., vol. 29, pt. 1, p. 189; Pl. 8, Figs. 12–14; 1904) obtained the second Recent species, S. cygnicollis, which he remarked recalled Ctiloceras in its conspicuous ringvarix.
Strebloceras hinemoa n. sp. (Text-Fig. 1).
Shell very small, a simple conical twisted tube, closed posteriorly by the spiral nucleus. The latter is of two whorls, very glossy, planorbid and regularly increasing, not ending in a varix, but marked off by a fine groove from the adult shell, which appears to be fitted into the embryo rather than directly prolonged from it. Tube becoming suddenly a little wider just after leaving embryo, thence regularly slightly expanding to aperture; glossy, but of different texture from embryo, unsculptured except for dense growth lines, quite irregularly one or two of these may be more prominent, and form a minute groove. Tube slightly bisinuous; if the embryo is placed in a horizontal plane, the tube curves first regularly forwards in this plane as if rapidly uncoiling, then bends downwards at about 30 degrees, then finally while continuing downwards takes a slightly backwards direction. Aperture thin, simple, sharp, circular; decidedly oblique to axis of tube at that point (the inner side of tube projecting fur-
![Thumbnail: [Volume 62, 1931-32 page 22]](/journals/rsnz/images/rsnz_62/thumbnails/rsnz_62_00_0068_0022_ac_02.gif)
ther), cutting it at about 75 degrees, so that the final plane of aperture is at about 45 degrees to the planes of both the embryo and a vertical section through it.
Length, 2.3 mm.; width of aperture, 0.4 mm.
Locality—Pukeuri, sandy clays in road-cutting (Awamoan), two shells obtained by sieving matrix in water.
Type in Finlay collection.
Fig. 1.—Strebloceras hinemoa n. sp.: Holotype. × 24.
The Recent S. subannulatum de Folin is larger (3 × 0.5 mm.), has a considerably greater twist, enlarges much more rapidly, and is ornamented with regular distant rings. S. cygnicollis Hedley is still more unlike, being very slender (3.35 × 0.45 mm.), with strongly marked growth rings anteriorly, aperture nearly at right-angles to plane of embryo, which does not end at the planorbid stage, but continues for some distance as an irregular swollen tube, marked, off from the adult shell by a very conspicuous trumpet-like varix. The Tertiary forms, as one might expect, seem nearest to the New Zealand species, and in the absence of good figures, it is difficult to give separative characters, but solutum and bezanconi seem at once distinct in their long slender shell and minute nucleus. Perhaps cornuoides and edwardsi are most like hinemoa, but they come from lower horizons, and are almost certainly different specifically.
The genus seems to have a most curious distribution in view of the paucity of its species. Perhaps these are not so rare as is thought, but are so minute and fragile as to be easily overlooked, destroyed, or mistaken for true Caecum.
I am grateful to Mrs. R. S. Allan for drawing the figure here reproduced.