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Arcidae.
Bathyarca bellatula n.sp. (Plate 23; Figs. 14, 15.)
Shell very small, highly inequilateral beaks at anterior third Hinge straight, nearly as long as the shell. Sculpture of about 30 narrow high radial ribs, with wider, flat interspaces, some of which on the middle and anterior of the disc develop a thin interstitial rib; both ribs and interspaces are crossed by thin, distant concentric ridges, which are not so high as the ribs except near the dorsal margin. Hinge with five anterior and 10 posterior teeth, the distal ones oblique, the medial ones very short and vertical. Shell margin crenulate within.
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Height, 2.75 mm.; length, 3.6 mm.; inflation (1 valve), 1.2 mm.
Locality: Hampden Beach. (Bortonian Stage.)
In his valuable revision of the generic divisions of the Arcidae, P. W. Reinhart (1935, p. 36) has correctly pointed out that the two New Zealand fossils Microcucullaea crenulifera and M. pectinata Marwick, 1931, are closer to Bathyarca than to Microcucullaea. His placing of Microcucullaea as related to Bathyarca rather than to Cucullaea is also justified by a general consideration of the hinges, sculpture and shape of these small shells.
In his table of distinguishing characters, Reinhart gives Bathyarca as having a smooth margin, but this seems to be a variable character, as regards the present scope of the genus. A great range of differences in hinge teeth is also shown, indeed it seems certain that some very distantly related species are at present included. Reinhart stated that he had not found a definite record of Bathyarca earlier than the Oligocene, apparently disregarding Cossman's suggestion, cited by Woodring (1925, p. 53), that A. lissa Bayan from the Parisian Eocene is a Bathyarca. This absence or rarity of the genus in the Eocene gives added interest to B. bellatula.