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Rissoinidae.
Genus Keilostoma Deshayes.
1850. Traité élém. Conch., Atlas, p. 46. = Paryphostoma Bayan, 1873. Genotype (by monotypy): Melania marginata Lamk. (= Bulimus turricula Brug.) Lutetian, Paris Basin.
Keilostoma malingi n.sp. (Plate 24; Figs. 17, 18.)
Shell rather small, turriculate, imperforate. Protoconch not preserved on any of the specimens. Whorls with almost straight sides, suture plainly but not strongly marked, faintly staging the spire. Sculpture of flat, imbricate bands also faintly staging the spire, six on early whorls increasing to seven on later ones; body-whorl with 10 or 11 broad bands and a few narrow ones anteriorly. Aperture, entire, narrowly channelled posteriorly, broadly subtruncate anteriorly; peristome continuous. Outer lip gently convex, slightly reflexed and thickly callused, callus growing forward considerably. Inner lip thickly padded.
Height (incomplete), 12.5 mm.; diameter, 4.3 mm.
Locality: Opua River, a quarter mile below Gorge Bridge, Kakahu (Bortonian), collected by P. B. Maling.
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K. malingi rather closely resembles the French and English Eocene K. minus (Desh.) in size, shape and sculpture, differing in having a straighter outline, slightly broader spirals, weaker growth lines and a greater spread of callus on the base. It agrees much more closely with minus than with any of the three Cretaceous Indian species, substriatum, subulatum, and politum. It is probably, therefore, directly related to the European species and so forms a useful link in connecting the Bortonian stage with the Eocene.
Cox (1931, p. 47) has described a Keilostoma subturricula from the Laki (Eocene) of India, but it was formerly identified as the European turricula Brug., and is not so close to malingi as is minus.
The generic term Paryphostoma was used for this shell by Finlay and Marwick (1940, p. 87), following Cossmann (1921, p. 70) who considered Keilostoma preoccupied by Chilostoma Fitz., 1833. Sherborn also cites a Cheilostoma Diesing, 1850, that is even closer in form, but may not be earlier. On reconsideration, the writer has decided, in view of the vagueness of the rules on this subject, to use the older name, Keilostoma.