Bentharca irregularis sp. nov.

Figures 50-54

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

Type and material.—Holotype: RM19382a, a right valve, from the bottom sediments of "Shodokutsu" of Ie Islet, Okinawa. Paratypes: RM19380 (dead) from "Witch's House", RM19381 (dead) from "Lunch Hole" of Shimoji-Irabu Islet. RM 19382 (dead) from the type locality. Living specimens have not been found.

Diagnosis.—Small-sized, transversely elongated, weakly carinated species of Bentharca, characterized by not much reduced anterior part, weakly sulcated central part of flank, relatively weak commarginal lamellae, often bifurcated radial ribs, weak marginal crenulation, relatively small D-shaped Pd I, and distinct Pd II.

Description.—Shell rarely exceeds 3.0 mm in length, transversely elongated, roundly subtrapezoidal but somewhat irregular in outline, moderate in shell thickness, about 2.0 times longer than high. Posterior carina relatively weak and rounded; posterior area not much depressed. Posterior margin nearly straight, passing gradually into venter. Ventral margin commonly indented in the middle but sometimes nearly straight. Umbo large, orthogyrous, situated at one-fourth of dorsal margin from anterior end. Flank and posterior area with numerous, often bifurcated radial ribs and relatively weak commarginal lamellae, tuberculated at their intersections. Radial ribs commonly weakened in the middle part of the flank. Marginal crenulation indistinct. Ligament area moderate in breadth, vertically striated. Dentition consists of two or three small anterior and five or more short opisthoclinal posterior teeth. Pd I Dshaped, ranging 101-151 µm in maximum diameter; Pd II clearly demarcated, veliconch (Pd I +Pd II) ranging 174-269 µm in maximum diameter.

Remarks.—The present species is probably closely related to Bentharca tenuis sp. nov. in view of the outline, surface sculpture and hinge structure, but the umbo is larger, the radial ribs are more strongly tuberculated and the Pd I is much smaller. It is easily distinguishable from other cave arcids by the weaker posterior carina, the not much reduced anterior part, weaker commarginal lamellae and clearly demarcated Pd II (Figure 54). In the outline the present species is also somewhat similar to Bentharca xenophoricola (Kuroda), but differs from that species in its much smaller size, strongly tuberculated radial ribs and not much reduced anterior part. Because of the presence of well-developed Pd II suggesting planktotrophic development and the ignorance of ultimate shell size, it is not convincing that the present species is really indigenous to eavernicolous environment.

Distribution.—Common in a sublittoral cave of Ie Islet, and rare in a few sublittoral caves of Shimoji and Irabu Islets, Ryukyu Islands.