GNS Science

Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)

New Zealand Cenozoic Mollusca

Spinomelon thomsoni (Marwick, 1926)


(Pl. 6n): GS5467, J39/f7508, Pareora River, near Evan's Crossing, Bortonian (GNS)

Beu & Maxwell (1990): Chapter 7; p. 110; pl. 6 n.

Synonymy: Waihaoia thomsoni Marwick, 1926a, p. 277; Beu & Maxwell 1990, p. 110, pl. 6 n; Spinomelon thomsoni, Maxwell (1992, p. 140; 2009)

Classification: Volutidae: Zidoninae

Description: Size small for genus (height 52-62 mm), narrowly fusiform, spire 0.4 total height. Protoconch not known. Teleoconch of about 6 or 7 whorls, early whorls convex, later whorls broadly shouldered near middle, angulation increasing in prominence for 2 or 3 whorls then gradually weakening, becoming obsolete on last whorl of some shells; last whorl elongate, broadly excavated with a short, slightly curved neck. Axial sculpture commencing as narrowly rounded costae with much broader interspaces, 8-9 per whorl, reaching from suture to suture, becoming thickened at shoulder angulation on later whorls, in some shells reduced to peripheral nodules on last whorl, in others becoming obsolete on last whorl. Spiral sculpture of very numerous, exceedingly fine striae, retained only on unabraded parts of shell. Aperture narrow, elongate; columella gently convex, bearing 4 or 5 narrow, well spaced plaits. Siphonal notch shallow, fasciole low, rounded. Outer lip thin.

Comparison: Distinguished from Spinomelon allani (Pl. 8p) by its more slender shell and in having the axial costae changing into peripheral nodules or becoming obsolete on the last whorl. Although Marwick (1926) and Finlay considered the earlier, relatively small species assigned to Waihaoia to be generically distinct from later, larger species assigned to Spinomelon, the protoconchs of Eocene species all have a small, narrow spine on the apex, and the Eocene species seem merely to have been earlier ones in a genus that increased in size with time. Waihaoia Marwick, 1926 was regarded as a synonym of Spinomelon Marwick, 1926 by Maxwell (1992, p. 140).

Distribution: Bortonian, Waihao Greensand, Waihao Downs, probably South Branch, Waihao River (type); Pareora River (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
ISBN 978-0-478-19705-1
ISSN 1177-2441
(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)


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