Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 9v): GS13416, G45/f61, Pomahaka River, 500 m downstream from Oyster Creek, Duntroonian? (GNS)
(Pl. 9w): GS13416, G45/f61, Pomahaka River, 500 m downstream from Oyster Creek, Duntroonian? (GNS)
Beu & Maxwell (1990): Chapter 10; p. 141; pl. 9 v, w.
Synonymy: Fusus plicatilis Hutton, 1873b, p. 3 (not Fusus plicatilis Bean MS, in synonymy of Clavatula turricula (Montfort) in Wood 1848, p. 62); Pomahakia aberrans Finlay 1927b, p. 507 (new name for Fusus plicatilis Hutton, preoccupied)
Type species of Pomahakia Finlay, 1927
Description: Rather small for family (height 18-25 mm), bucciniform, spire 0.4 total height. Protoconch mamillate, of 1.5 strongly convex, smooth whorls with bulbous initial whorl. Teleoconch of 5-6 whorls shouldered somewhat above middle on spire, shoulder becoming rounded on last whorl of some shells but remaining angled on others; last whorl with evenly rounded periphery or weak basal subangulation. Axial sculpture, apart from growth lines, commencing as narrow, opisthocyrt costae reaching from suture to suture, but becoming obsolete on ramp and more widely spaced during growth, reduced to peripheral nodules on last whorl, or becoming obsolete on last few whorls; 11-12 per whorl. Spiral sculpture on spire of very subdued, narrow, rounded cords and threads, last whorl with much more prominent but nonetheless subdued, rounded or flattened cords. Shell with distinctly glazed appearance. Aperture ovate with a short, shallowly notched siphonal canal; fasciole rounded, not prominent. Inner lip moderately callused, outer lip thin with a broad, shallow sinus with its apex on shoulder angulation.
Comparison: Pomahakia is a monotypic genus of uncertain affinity but probably referable to the Buccinulidae. P. aberrans is superficially similar to some species of Aethocola, but differs in having a mamillate rather than a conical protoconch, and in having a shallow but distinct sinus in the outer lip. It may represent an offshoot from Aethocola or a related group that became adapted to a brackish-water habitat. The glazed exterior suggests that Pomahakia aberrans had a reasonably thick periostracum when alive. A few species of tropical Pacific fresh-water shells with heavy brown periostraca (in the genera Clea A. Adams, 1855 and Anentome Cossmann, 1901) have siphonal canals and are now referred to the Buccinidae, although they were previously referred to the Thiaridae, and this suggests an alternative position for Pomahakia.
Finlay (1927b, p. 507) considered that Fusus plicatilis Hutton is preoccupied by "Fusus plicatilis Bean MS", published as a synonym of Clavatula turricula (Montfort) by Wood (1848, p. 62). Contrary to common sense, we are unfortunately obliged to use Finlay's replacement name as Article 11.6.1 of the International Code of Zoological Nomenclature (1999 ed.) states that a name published in synonymy is to be regarded as available if prior to 1961 it has been treated as a senior homonym, although it dates from Wood (1848).
Distribution: Duntroonian(?), Pomahaka Formation, Pomahaka River (type); Waikoikoi Stream. Uncommon.
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)