Genus Isolimea Iredale, 1929

Isolimea limopsis (Nomura and Zinbo, 1934)

Figures 247-252

1934. Lima (Limea) limopsis Nomura and Zinbo, Sci. Rep. Tohoku Imp. Univ., 2nd ser., vol.16, no.2, p.154, pl.5, figs.11-12.

1943, Limatula (Notolimea) limopsis (Nomura and Zinbo), Oyama, Conchologia Asiatica, vol.1, p.l2, pl.1, figs.4-5.

1977. Isolimea limopsis (Nomura and Zinbo), Habe, Systematics of Mollusca in Japan, Bivalvia and Scaphopoda, p. 103.

A few empty valves from the bottom sediments of "Fool's Palace" (RM19498) of Shimoji Islet are referable to Lima (Limed) limopsis Nomura and Zinbo, 1934, which was later referred to Isolimea by Habe (1977a). Lamprell and Whitehead (1992) treated the type species of Isolimea, Limea parviila Verco, 1908, as belonging to Gemellima which was proposed in the same paper by Iredale (1929). Though the distinction between Isolimea and Gemellima at the generic or subgeneric level may be controversial, the present species seems to be more closely related to the type species of Isolimea than that of Gemellima.

The shell is small, thick, higher than long, not exceeding 5.0 mm in height, subovate and opisthocline, characterized by several oblique denticles of Limea-type on the anterior and posterior parts of the hinge plate and about 20 radial ribs which are much narrower than their intervals. Ventral margin coarsely crenulated in accordance with radial ribs. Though the umbonal area is somewhat abraded in every observed specimen, Pd I may be about 170 µm in diameter, and Pd II seems to be absent.

The present species was first described by Nomura and Zinbo from the Pleistocene Ryukyu Limestone at Kikai Island, Kagoshima Prefecture, and later recorded from lower sublittoral substrata between Yaku Island and Miyako Islands (Habe, 1977a).Because only a few specimens have been found in the cave sediments, it is rather unlikely that the habitat is confined to cavernicolous environments.