Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 4n): holotype, GS1177, CH/f3, coast north of Tioriori, Chatham Island, Waipawan (?) (TM6847, GNS)
Beu & Maxwell (1990): Chapter 6; p. 96; pl. 4 n.
Synonymy: Cirsotrema (Tioria) youngi Marwick 1928, p. 484
Type species of Tioria Marwick, 1928
Description: Size moderate for family (height c. 25 mm). Protoconch not known; teleoconch of 6 or 7 whorls, shouldered at about 0.8 whorl height on spire; last whorl with prominent basal disc. Axial sculpture of prominent, slightly prosocline lamellae, reaching from suture to suture on spire, produced into small triangular spines on shoulder angle, and reaching across base to fasciole. On base, crests of lamellae are folded over towards aperture. Penultimate whorl with 21-28 lamellae. No definite varices. Spiral sculpture of very fine grooves with low, flattened interspaces, between axial lamellae and extending onto their back (abapertural) sides. Umbilicus narrow, apparently almost entirely closed in adult. Aperture circular, peristome continuous, outer lip with a small spine at shoulder angle and another at junction with fasciole.
Comparison: Cirsotrema youngi is the only gastropod recorded from this unit, probably becauseunlike most gastropods it has a calcitic rather than an aragonitic shell and so has escaped diagenetic solution. Most other New Zealand species of Cirsotrema have a similar mode of occurrence. Tioria differs from typical Cirsotrema mainly in lacking distinct varices, in having the crests of the basal lamellae turned over and in having the fasciole separated from the inner lip. Although two additional species have been referred to Cirsotrema (Tioria), viz. C. (Tioria) marshalli (Bortonian, Hampden) and C. (Tioria) forresti Dell, 1956 (Recent, Hawke's Bay), neither seems to be closely related to C. youngi, and they are better referred to Cirsotrema (sensu stricto).
Distribution: Waipawan (?), Tutuiri Greensand, Tioriori, northern Chatham Island (type) (rare).
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)