Cardtta uruma sp. nov.
Figures 275-283
Type and material.— Holotype: RM19523a, a living specimen, from the bottom sediments of "Cross Hole" of Irabu Islet, Miyako Islands. Paratypes: RM19521 (dead) from "Devil's Palace", RM19522 (living) from "Fool's Palace", RM19608 (living) from "Toriike" of Shimoji Islet. RM19523 (living) from the type locality of Irabu Islet. RM19524 (living) from "Shodokutsu" of Ie Islet.
Diagnosis.— Small-sized, trapezoidal, relatively short and brownish species of Cardita, characterized by about 20 spiniferous radial ribs, very strong shell convexity, long posterior cardinal tooth in each valve, and strongly crenulated inner margin.
Description.— Shell rarely exceeds 5.0 mm in length, commonly yellowish with brownish posterior part, trapezoidal, relatively short, strongly inflated, weakly carinate, with reduced anterior part and broadened posterior part. Umbo very prosogyrous, incurved, situated very anteriorly. Antero-dorsal margin short, deeply concave, while postero-dorsal margin is long, forming an obtuse angle with posterior margin. Ventral margin almost straight or a little concave to form byssal indentation. Surface ornamented with about 20 strong radial ribs on which a number of long halftubular spines occur. Ligament opisthodetic, external. Dentition of Cardita-type, formulated as: AI 3a 3b PI/AII 2 4b PII; 3a and 2 short, subparallel to pre-umbonal margin, 3b and 4b long, very oblique, lateral teeth remote and rather weak, Pd I Dshaped, moderate in size, about 120 µm in maximum diameter. Veliconch (PdI + PdII) about 260 µm in diameter.
Remarks.— The present species has been found alive mainly on the cave walls and ceilings. In view of the external features and hinge structure the present species is undoubtedly referable to Cardita. It seems to be characterized by a relatively short shell, small shell size, strongly crenulated inner margin and about 20 spinose radial ribs. The spines are almost identical with those of the Japanese species generally referred to Cardita nodulosa Lamarck, 1819, but the shell is much smaller and shorter, and the radial ribs are more numerous in the present species.
Distribution.— This species occurs, though rather rarely, in some sublittoral caves of Ie, Shimoji and Irabu Islets, Ryukyu Islands.