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Subgenus Hyphantosoma Dall.
1902. Proc. U.S. Nat. Mus., vol. 26, p. 354.
Type (by original designation): Cytherea carbasea Guppy, Miocene, Jamaica.
Figured: Woodring, Miocene Moll. Bowden, Pelecypods. and Scaphopods, Pl. 20, figs. 15–19.
Pitar (Hyphantosoma) sculpturatus (Marshall). Plate 10, fig. 8, 11
1918. Macrocallista sculpturata Marshall, Trans. N.Z. Inst., vol. 50, p. 272, pl. 21, figs, 6, a.
1927. Pitar (Hyphantosoma) sculpturatus (Marsh.); Marwick, Trans. N.Z. Inst., vol. 57, p. 594, figs. 74–76.
? Miocene: G.S. loc. 800, west slope of Puketoi Range opposite Ormondville.
Oligocene: Pakaurangi Point (Hutchinsonian).
The left hinge has not yet been seen, the four specimens known from Pakaurangi Point being right valves and the single one from loc. 800 a closed individual. The New Zealand species differs from the genotype, carbaseus, in having a somewhat wider right posterior cardinal and a longer groove leading to the anterior lateral pit, which is thus more distant from the cardinals. The ornamentation of sculpturatus is considerably weaker than that of carbaseus, being intermediate between this species and opisthogrammatus Dall. These differences, however, do not appear to have more than specific value, especially in view of the close agreement of the other characters, even to the confluence of the pedal retractor.
No satisfactory explanation has yet been put forward as to how this peculiarly sculptured West Indian group came to inhabit New Zealand seas during the Oligocene-Miocene, and the possibility of sculpturatus being a parallel development from the Fossacallista stock must be considered. No likely ancestors are known in New Zealand, but the greatly inflated N. opima from the Balcombian of Victoria, though smaller, has a similar shape and hinge. Its lateral pit, however, is close to the cardinals and, of course, it shows no trace of zig-zag sculpture. Consequently it presents no positive evidence that sculpturatus arose from this kind of shell.
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Possibly the left valve of sculpturatus, when found, will supply additional evidence of origin. The left posterior cardinal of carbaseus is quite separate from the nymph and diverges noticeably from it with age. Corresponding to this, in the right valve, the posterior cardinal is narrow and more than its own width distant from the nymph. On the other hand, the right posterior cardinal of sculpturatus is relatively broad, and the space between it and the nymph is narrow. This suggests that the left posterior cardinal of sculpturatus is closely connected with the nymph and favours derivation from the Fossacallista stock. The right anterior hinge-plate of carbaseus is more excavated than that of sculpturatus and the lateral teeth are weaker; but, admittedly, the differences are not great, and it seems advisable to regard sculpturatus as a true Hyphantosoma until a definite case to the contrary is established.
In this connexion it should be remembered that New Zealand has a few other faunal contacts with the southern part of North America. Curiously enough, two outstanding examples are also Veneridæ, namely, Chione str. and Protothaca, to which stocks the New Zealand groups Austrovenus and Tuangia are closely related, neither being represented in Australia or the Old World.