Urumella concava sp. nov.

Figures 159-166

Type and material.—Holotype: RM19448a, a right valve, from the bottom sediments of "L-arch" of Irabu Islet, Miyako Islands. Paratypes: RM19442 (dead) from "Devil's Palace", RM19443 (dead) from "Fool's Palace", RM19444 (dead) from "Witch's House", RM19593 (dead) from "Wall Cave", RM19445 (dead) from "Toriike" of Shimoji Islet, RM19446 (partly living) from "Lunch Hole", RM19447 (dead) from "W-arch", RM19448 (dead) from the type locality, RM19449 (dead) from "Cross Hole" of Irabu Islet. No living specimens have been found except two juveniles from "Lunch Hole".

Diagnosis.—Very small-sized, uniquely distorted mytiliform species, characterized by very prosoclinal growth axis, remarkably concave posterior margin (only in adult stage), subcentrally placed umbo, densely spaced commarginal lamellae, thin filmy periostracum, numerous divergent riblets on posterior area and persistent denticles of provinculum.

Description.—Shell very small, rarely exceeding 1.3 mm in length, translucent, àrgenteous, very prosoclinal, roundly carinate, much longer than high, strongly inflated. Periostracum thin, filmy, tinged with yellow. Dorsal margin straight, relatively short; anterior margin rounded, passing gradually into venter which is feebly indented in the middle; posterior margin remarkably concave in its lower half where the shell is strongly incurved, though this feature appears only in adult growth stage. Umbo slightly prosogyrous, located a little anteriorly from mid-point of dorsal margin. Surface wholly covered with fine regular commarginal lamellae; posterior area behind carina also marked with 20 or more curved radiating riblets which perpendicularly cross commarginal lamellae and produce minute tubercles at intersections. A few similar riblets occasionally observed on antero-dorsal area. No marginal crenulations. Primary ligament pit low-triangular, situated just below umbo, persistent throughout growth. Secondary outer ligament undeveloped. Denticles of provinculum persistent until latest growth stage; anterior series numbers about 10, nearly vertical, relatively short; posterior series numbers about 16, becoming gradually longer and more opisthocline toward posterior. Adult teeth not developed at all. Pd I large, D-shaped, about 160 µm in maximum diameter; Pd II not discriminated.

Remarks.—Several specimens are articulated and very fresh, bearing filmy periostracum. None of them, however, is living. The shape of the valves is so unique that no described species of mytilids is comparable with the present species. The concave and incurved posterior margin is not a malformed product because it is observed in all the adult specimens.

Distribution.—Common in many sublittoral caves of Shimoji and Irabu Islets, Miyako Islands. No specimen has been found in the cave of Ie Islet.