Genus Sinum Roeding, 1798.
Type Helix haliotoidea Linné.
Sinum marwicki n. sp. (Figs. 1, 4).
Shell small, greatly depressed; whorls nearly three including a smooth, planorboid protoconch of one and a-half whorls; last whorl enlarging fairly rapidly; apex excentric and nearer front edge; spire flat, about one-fifth height of shell, and dorsal surface convex; body whorl lightly excavated ventrally between inner edge of aperture and outer margin of base, the concavity becoming more marked towards the small, partly hidden umbilicus. The upper surface has slightly undulating, somewhat flattened spiral threads, about 35 in number (3 per mm.), separated by grooves slightly wider than the ridges. Towards the periphery the last ten or so lirae suddenly become finer and the width of the interstices less in relation to that of the ridges. These are crossed by well-defined convex growth-lines. No spirals are developed on the base, but the lines of growth are prominent as they sweep convergingly into the umbilical tract. Suture markedly tangential. Aperture large, circular, nearly two-thirds greatest diameter of shell, angled above. Outer lip thin and strongly convex; inner lip covering parietal wall (callus partially broken away in specimen), reflexed and partly hiding umbilicus.
Height, 5 mm.; greatest diameter, 14 mm.; least, 11 mm.
Locality—White Rock River shell bed (Awamoan), South Canterbury.
Holotype (the only specimen) in the writer's collection.
This is the third species of Sinum s. str. described from the Tertiary of New Zealand. It is not unlike S. infirmum Marwick from the Awamoan beds of Ardgowan and Pukeuri, but is readily separable on account of its less excentric apex and more compressed character, causing a sharper periphery to the body whorl, which also enlarges less rapidly in the new species than it does in S. infirmum.*
[Footnote] * Since the above was written the writer has collected a perfectly preserved topotype of S. infirmum Marwick, its dimensions being slightly less than those given for the holotype.