Cosa kinjoi sp. nov.
Figures 84-92
1992. Cosa sp. B, Kase and Hayami, Jour. Moll. Studies, vol.58, p.448, listed.
Type and material.—Holotype: RM19586a, living specimen, from the cave wall of "Coral Hole" of Shimoji Islet, Miyako Islands. Paratypes: RM19404 (dead) from "Witch's House", RM19585 (dead) from "Toriike", RM19405 (dead) from "Black Hole", RM19586 (living) from "Coral Hole" of Shimoji Islet. RM19406 (dead) from "W-arch", RM19407 (dead) from "L-arch", RM19408 (dead) from "Cross Hole" of Irabu Islet. RM19409 (living) from "Shodokutsu", RM19568 (dead) from "Daidokutsu" of Ie Islet.
Diagnosis.—Very small-sized, wedge-like trapezoidal species of Cosa, characterized by thin periostracum, projected anterior part of shell, fine but sharp commarginal lamellae, generally undeveloped radial ribs, entire inner margin, and anteriorly located, large and D-shaped Pd I.
Description.—Shell very small, scarcely exceeding 1.2 mm in length and height, wedge-like trapezoidal in outline, expanded anteriorly and postero-ventrally, roundly carinate. Periostracum thin, not hairy. Dorsal margin moderate in length; anterior margin short, deeply concave; posterior margin nearly straight. Umbo slightly prosogyrous, located anteriorly. Surface commonly marked only with dense but sharp commarginal lamellae, although sometimes (especially in samples from Shimoji Islet) several obscure radial ribs are developed. No marginal crenulations. Ligament pit small, subquadrate, slightly prosocline, located about one-fourth of dorsal margin from anterior end. Provinculum persistent until the latest growth stage, consisting of about 8 anterior and about 23 posterior subvertical denticles. Adult teeth absent. Pd I large, smooth, ranging 235-263 µm, D-shaped with no marginal brim, but having a small tubercular boss near dorsal margin. Pd II absent.
Remarks.—In many caves the present species is represented by empty shells, but in "Coral Hole" of Shimoji Islet and "Shodokutsu" of Ie Islet some individuals have been found alive mainly on the cave walls. Unlike Cosa waikikia, the periostracum is rather thin and not hairy.
Such normally shaped (not hat-shaped) Pd I may be unfamiliar in the genus Cosa, though there is a small central boss near the dorsal margin (Figures 86, 88). We think that the size and shape of prodissoconch is generally stable in a population but consid-erably plastic in one and the same genus. Fine and sharp commarginal striae and an obliquely expanded wedge-shaped outline also characterize the present species. Most specimens from the type locality are free from radial ribs, whereas the samples from the caves of Shimoji Islet exhibit a wide range of variation hi the development of radial sculpture.
The species name is dedicated to Mr. S. Kinjo, the chief of Ie SCUBA Diving Service, who kindly provided us various facilities during our field work.
Distribution.—Common in several sublittoral caves of Ie, Shimoji and Irabu Islets, Ryukyu Islands. According to our preliminary survey, this species also occurs in "Balicasag cave" off Panglao Islet of Bohol Island, the Philippines, and "Chandelier cave" near Koror, Palau Islands.