Cosa uchimae sp. nov.
Figures 93-99
1992. Cosa sp. C, Kase and Hay ami, four. Moll. Studies, vol.58, p.448, listed.
Type and material.—Holotype: RM19410a, a right valve from the bottom sedi-ments of "Shodokutsu" of Ie Islet, Okinawa. Paratypes: RM19410 from the same locality. No living specimen has been found.
Diagnosis.—Very small-sized, subovate species of Cosa characterized by short dorsal margin, absence of radial ribs, weak commarginal lamellae, entire inner margin, relatively short denticles of provinculum, and large hat-shaped Pd I which has a wide smooth brim.
Description.—Shell very small, scarcely exceeding 1.0 mm in length and height, subovate rather than quadrate, not carinate, with relatively weak convexity. Dorsal margin unusually short; anterior margin slightly concave; posterior and ventral mar-gins hardly discriminated from each other. Umbo orthogyrous, situated anteriorly. Surface marked only with weak commarginal lamellae. Neither radial ribs nor mar-ginal crenulations. Ligament pit triangular, acline, located about one-third of dorsal margin from anterior end. Denticles of provinculum persist until the latest growth stage but comparatively short; anterior and posterior series consist of about 11 and 13 denticles, respectively. Adult teeth do not appear. Pd I large, ranging 197-228 µm in maximum diameter, hat-shaped with round-topped central boss, gentle foot and non-frilled brim margin; Pd II absent.
Remarks.—The present species can be discriminated from other cave philobryids by its smaller adult size, shorter dorsal margin, subovate outline, absence of radial ornament, and weak commarginal lamellae, and the gentle foot of volcano-like Pd I. It shares undeveloped radial ribs with Cosa kinjoi, sp. nov., but is easily distinguishable from that species by the weaker shell convexity, shorter dorsal margin, undeveloped commarginal lamellae, shorter denticles of provinculum and hat-shaped Pd I.
The species name is dedicated to Miss M. Uchima, who kindly assisted us in our field work at Ie Islet in various ways.
Distribution.—Common in a sublittoral cave of Ie Islet, Ryukyu Islands. The present species has not been found in any cave of Miyako Islands.