Cratis kanekoi sp. nov.
1984. Cosa sp., Kaneko, Explanation of 11th special exhibition, Osaka City Museum of Natural History, p.23, pl.8, figs.6-7 (clearly illustrated but with no verbal description).
1991. Philobryidae gen. and sp. Ill, Kaneko, Venus, vol. 50, p. 158, fig. III.
Type and material.—Holotype RM 19411a, a left valve, from the bottom sediments of "Devil's Palace" of Shimoji Islet, Miyako Islands. Paratypes: RM 19411 (dead) from the type locality, RM19412 (dead) from "Cross Hole" of Shimoji-Irabu Islet. No living specimens have been found.
Diagnosis.—Small-sized, tall, thick and polygonal species of Cosa, characterized by about eight beaded radial ribs, marginal crenulations restricted to postero-ventral portion, prosoclinal ligament pit, development of an adult tooth in anterior hinge plate in each valve, and hat-shaped Pd I with elevated and coarsely frilled brim margin.
Description.—Shell small, scarcely exceeding 1.7 mm in height, about 1.3 times as high as long, weakly carinated. Test considerably thick. Dorsal margin relatively short; anterior margin nearly straight; posterior and ventral margins polygonally angulate in accordance with external radial ribs. Umbo orthogyrous, located anteriorly. Surface ornamented with regular commarginal lamellae and about eight strong and beaded radial ribs; each interval of radials broadly concave. Marginal crenulations absent except for a few at postero-ventral portion. Ligament pit small, triangular, considerably prosoclinal, located about one-fourth of dorsal margin from anterior end. Provinculum persistent until the latest stage, consisting of about 8 anterior and about 15 posterior subvertical denticles. Adult hinge plate more or less developed in front of ligament pit, commonly producing a large tubercular adult tooth and a socket ill each valve. Pd I large, hat-shaped, about 250 µm in maximum diameter, characterized by low conical central boss and thickly elevated and coarsely frilled brim margin.
Remarks.—The present species is well characterized by a few widely spaced and beaded radial ribs, polygonal valve margin and the appearance of an adult tooth on the anterior hinge plate. A large socket is present behind the tooth in LV (Figure 105) and in front of the tooth in RV (Figure 110). Though the surface sculpture is similar to that of some species of Cosa, the presence of a large adult tooth in each valve indicates that the present species is better placed in the genus Gratis.
The present species is firmly identical with Cosa sp. by Kaneko (1984) and from Philobryidae gen. and sp. III from dredging sands at Nakagusuku-son, Okinawa, Hence the new species name. In view of the polygonal outline and strong radial ornaments, this species may be somewhat similar to Hochstetteria costata Bernard, 1896, the type species of Cosa, from Stewart Island, New Zealand. However, it differs from that species in its developed anterior adult teeth, less prosoclinal shell and much larger antero-dorsal angle. It may be closer to Cosa serratocostata serratocostata Powell, 1933, and Cosa serratocostata dispar Powell, 1937, from New Zealand, but differs from them in the fewer radial ribs and shorter dorsal margin. Philobrya tatei Hedley, 1901, from New South Wales, shows similar outline and hinge structure, but the radial ribs on the posterior area are finer and more numerous than those of the present species.
Distribution.—Rare in a few sublittoral cave of Shimoji and Irabu Islets, Ryukyu Islands. Kaneko (1984) recorded this species from the dredged sands from the sea bottom of the Pacific coast of southern Okinawa. According to Dell (personal commu-nication, March 25, 1993), the same species occurs in a number of areas in the south-western Pacific. Therefore, it is unlikely that the habitat of the present species is restricted to cryptic environments.