Cratis ohashii sp. nov.

Figures 111-120

Compare 1991. Philobryidae sp. IV, Kaneko, Venus, vol.50, p.158, fig.IV.

1992. Cratis sp., Kase and Hayami,Jour. Moll. Studies, vol.58, p.448, listed.

1993. Cratis sp., Hayami and Kase, Univ. Mus. Univ. Tokyo, News, no.27, p.3, fig.4.

Type and material.—Holotype: RM19416a, a left valve, from the bottom sedi-ments of "Shodokutsu" of Ie Islet, Okinawa. Paratypes: RM19415 (dead) from "Fool's Palace" of Shimoji Islet, RM19416 (dead) from the type locality. No living specimens have been found.

Diagnosis.—Small-sized and subquadrate species of Cratis, characterized by incurved anterior margin, remarkably cancellate and beaded surface, one large anterior adult tooth and huge hat-shaped Pd I with mammilla-like central boss and flared brim margin.

Description.—Shell commonly smaller than 2.8 mm in height, roundly subquadrate, obscurely carinate, solid and strongly inflated. Ratio of height/length evidently increases with growth; about 1.0-1.2 in juvenile stage but larger than 1.3 in adult stage. Anterior peripheral area incurved, with nearly straight or slightly concave margin; posterior and ventral margins obscurely discriminated from each other. Umbo orthogyrous, located subcentrally in early growth stage but about one-fourth of dorsal margin from anterior end in adult. Surface marked with remarkably cancellate ornamentation consisting of regular commarginal lamellae and about 40 radial ribs, with numerous fine beads at their intersections. Inner margin weakly crenulated in accordance with radial ribs. Ligament pit triangular, slightly prosocline. Denticles of provinculum clearly observable along the dorsal margin of Pd I even in adult indi-vidual, but undeveloped along the hinge-line of dissoconch. Adult hinge teeth developed in later growth stages, commonly consisting of one subvertical anterior tooth and one or two opisthoclinal posterior teeth in each valve. Pd I extremely large, ranging 514-96 µm in maximum diameter, hat-shaped with mammilla-like central boss and widely flared subtriangular brim margin. Earliest part of dissoconch free from radials and marked only with faint commarginal lamellae.

Remarks.—This species is not much different from Gratis progressa Hedley, 1915, the type-species of this genus, from 100 fathoms off Port Macquarie, New South Wales. In view of the redescription by Tevesz (1977), however, the present species differs from the Australian species in its smaller size, less rounded (more quadrate) outline, incurved and concave anterior margin and more prominent posterior carina. In these respects it may be more closely related to Cratis retiaria Powell, 1937, from New Zealand, but the radial ribs are fewer and probably more strongly beaded in the present species.

According to Kaneko's personal communication (August 12, 1991), this species is possibly conspecific with his (1991) "Philobryidae sp. IV" from off Cape Shionomisaki (100 m deep) of Kii Peninsula, because the features of Pd I and dissoconch are very similar.

The species name is dedicated to Mr. S. Ohashi, a skilled professional diver, who cooperated in our field work in Okinawa and Miyako Islands and gave us invaluable information about cave organisms and submarine topography.

Distribution.—Common in a sublittoral cave of Ie Islet, Ryukyu Islands. The present species has been very rarely found from the caves of Shimoji Islet. Several specimens resembling the juveniles of this species have been found from the sedi-ments near Gyuchaku-iwa, Kumomi, Izu Peninsula, Shizuoka Prefecture, central Honshu.