Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 9r): GS13416, G45/f61, Pomahaka River, 500 m downstream from Oyster Creek, Duntroonian? (GNS)
Beu & Maxwell (1990): Chapter 10; p. 141; pl. 9 r.
Synonymy: Ancillaria (Amalda?) pomahaka Hutton 1873b, p. 6-7; Zemelanopsis pomahaka; Melanopsis (Stilospirula?) pomahaka, Beu & Maxwell 1990, p. 141
Description: Size moderate for family (height 15-40 mm), elongate-ovate, spire about 0.3-0.4 total height. Protoconch not clearly marked off from teleoconch, apparently mamillate, of about 1.5 whorls with a bulbous initial whorl. Teleoconch of 10-13, possibly more, whorls, first 9 or so increasing only slowly in diameter and flat-sided or feebly convex, so that apical whorls (where preserved) are narrowly conical; later whorls increasing quickly in size and becoming distinctly concave posteriorly and strongly convex below. No sculpture apart from weak growth lines and feeble spiral striae, shell somewhat polished. Aperture ovate, constricted posteriorly by heavy, triangular parietal callus pad alongside which runs a narrow groove. Columella short, strongly concave except for leftward twist at anterior end, defining a short, unnotched siphonal spout; young shells with narrow, oblique plait near posterior end, but absent from adult shells. Inner lip narrowly but quite thickly callused below parietal pad. Outer lip thin, straight and vertical from suture to near base of parietal pad, strongly convex below to base where it bends back to siphonal spout.
Comparison: Melanopsis pomahaka is very similar to M. trifasciata (Opoitian-Recent) but differs in having a slender rather than a broadly conic, blunt apex, and in having the outer lip strongly convex below the parietal pad rather than prosocline and straight or weakly concave. Beu & Maxwell (1990, p. 141) took these characters to indicate that M. pomahaka is closely related to M. proboscidea Deshayes, 1862 (Eocene, France), the type species of Stilospirula Rovereto, 1899, but Stilospirula is otherwise known only from the Eocene and Oligocene of Europe. We consider it much more likely that M. pomahaka is phylogenetically related to Indo-Pacific Melanopsis species, and is likely to be directly ancestral to M. trifasciata, and we tentatively return it to Melanopsis sensu stricto.
Distribution: Duntroonian(?), Pomahaka Formation, Pomahaka River (type); Waikoikoi Stream. Locally very common.
Cite this publication as: "A.G. Beu and J.I. Raine (2009). Revised
descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990). GNS
Science miscellaneous series no. 27."
© GNS Science, 2009
(Included with a PDF facsimile file copy of New Zealand Geological Survey Paleontological Bulletin 58 in CD version from: Publications Officer, GNS Science, P.O. Box 30368 Lower Hutt, New Zealand)