Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 21u): GS9520, J41/f8029, Awamoa Creek, Oamaru, Altonian (GNS)
Beu & Maxwell (1990): Chapter 11; p. 205; pl. 21 u.
Synonymy: Waihaoia (Pachymelon) amoriaformis Marwick 1926a, p. 282
Type species of Pachymelon Marwick, 1926
Classification: Volutidae: Zidoninae
Description: Size moderate for subfamily (height 66-75 mm), broadly fusiform, rather solid. Protoconch prominent, bulbous, of about 2.5 smooth whorls, apex bluntly pointed. Teleoconch of 4-4.5 whorls, convex at first, later bluntly shouldered with broad, concave sutural ramp; last whorl gradually contracted, unexcavated. Axial sculpture of low, rounded costae reaching from suture to suture on spire but becoming obsolete on ramp on penultimate whorl of some shells, and absent from part or all of last whorl; 17-27 costae per whorl. Aperture constricted posteriorly; anterior notch broad and shallow, fasciole inconspicuous. Columella oblique, almost straight, with 6-8 narrow plaits. Outer lip convex, thickened within.
Comparison: Pachymelon amoriaformis is readily distinguished from other Early Miocene volutes by its rather solid shell, its short spire, its very shallow anterior notch, and its exceptionally numerous columellar plaits. Several other fossil and extant volutes have been assigned to Pachymelon in the past, but none closely resembles P. amoriaformis, and all are here referred to Alcithoe (in part following the action of Dell (1978) who transferred modern Pachymelon species to Alcithoe), pending a much-needed revision of the New Zealand members of the family. This distinctive species with an unusually large number of columellar plaits, therefore, remains the one species referred to Pachymelon, throwing some doubt on whether the genus should be separated from Alcithoe.
Distribution: Altonian; Target Gully Shellbed, Oamaru (type); Ardgowan Shellbed; Awamoa Creek; Mt Harris. Rare at all localities.
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)