Genus Limatula Wood, 1839

Limatula kinjoi sp. nov.

Figures 253-259

1992. Limatula sp., Kase and Hayami, Jour. Moll. Studies, vol.58, p.447, figs.1G,H, p.448, listed.

Type and material.—Holotype: RM19506a, a living specimen, from the bottom sediments of "Cross Hole" of Irabu Islet, Miyako Islands. Paratypes: RM19499 (dead) from "Devil's Palace", RM19500 (dead) from "Fool's Palace", RM19501 (dead) from "Witch's House", RM19502 (dead) from "Toriike", RM19503 (dead) from "Black Hole" of Shimoji Islet. RM19504 (dead) from "Lunch Hole", RM19607 (partly living) from "W-arch", RM19505 (dead) from "L-arch", RM19506 (partly living) from the type locality of Irabu Islet. RM19507 (dead) from "Shodokutsu" of Ie Islet.

Diagnosis.— Small-sized species of Limatula, characterized by narrow cardinal area, divaricated ribs on early dissoconch, centrally strengthened and weakly tuberculated radial ribs on later dissoconch, persistent denticles of provinculum, and large hat-shaped Pd I.

Description.— Shell very small, rarely exceeding 3.0 mm in height, nearly equilateral in young stage but slightly opisthocline in adult, about 1.7 times higher than long (though the ratio is much smaller in juveniles), white and opaque, strongly inflated. Umbo orthogyrous, situated near midpoint of dorsal margin. Dorsal margin moderate in length, truncated by anterior and posterior margins. Both auricles moderately wide, sharply angulated. Early dissoconch marked with a few irregularly divaricate ribs; later dissoconch with coarser growth lamellae and 25-30 weakly tuberculated radial ribs which are somewhat strengthened and widely spaced in the central part. Cardinal area relatively narrow, provided with relatively small low-triangular ligament pit. Adult teeth undeveloped. Denticles of provinculum persistent until the late growth stage. Pd I, large, ranging 233-265 µm in maximum diameter, hat-shaped with depressed summit of central boss and somewhat thickened brim; Pd II absent.

Remarks.— The weakly tuberculated radial ribs, persistent provinculum and relatively short shell in juvenile stage may remind one of those of the subgenus Notolimea, but in the adult stage the present species shares many characters with Limatula (s. s.) species. In addition to the unusually small adult size, the irregularly divaricate sculpture on the early dissoconch (Figure 255) seems to characterize this species. The large hat-shaped Pd I (Figures 256, 259) is also unique, because such a feature has not been recorded in any species of Limatula. We presume that the drastic change of ornamentation in the early dissoconch indicates the releasing stage of incubated juveniles.

Oyama (1943) described hitherto known species of Notolimea and Limatula (s. s.) in the Indo-West Pacific region. Fleming (1978) listed about 150 named fossil and extant species of Limatula, He classified the Pacific and southern ocean species into four subgenera: Limatula (s. s.), Limatuletta Fleming, 1978, Stabilima Iredale, 1939, and Squamilima Fleming, 1939 [an objective synonym of Antarctolima Habe, 1977b]. The present material, however, is not referable to any species of these subgenera in view of the unique prodissoconch shape and early dissoconch sculpture.

This species is named after Mr. S. Kinjo, a shell-collecting skilled diver of Itoman City of Okinawa, who assisted our field work and supplied many samples for study.

Distribution.— Common in many sublittoral caves of Shimoji, Irabu and Ie Islets of the Ryukyu Islands.