Order Mytiloida Ferussac, 1822
Superfamily Mytiloidea Rafinesque, 1815
Family Mytilidae Rafinesque, 1815
Subfamily Mytilinae Rafinesque, 1815
Genus Brachidontes Swainson, 1840

Brachidontes sp. indet.

Figures 129-133

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

This species is now represented by several empty valves from the bottom sediments of "Fool's Palace" (RM19422), "Coral Hole" (RM19588) of Shimoji Islet, "L-arch" (RM19423) and "Cross Hole" (RM19424) of Irabu Islet, Miyako Islands and "Shodokutsu" (RM19425) of Ie Islet, Okinawa.

It is characterized by elongate-oval outline, non-terminal umbo, rounded anterior margin and numerous fine divaricate ribs, which cross densely spaced commarginal lamellae and give a cloth-like appearance to the later dissoconch. No subumbonal septum is observed. The dorsal margin is marked with numerous subvertical den-ticles. The inner valve margin is finely crenulated in accordance with the divaricate ribs. The Pd I is fairly large, ranging 101-148 µm; the veliconch (Pd I + Pd II) is 206-292 µm in maximum diameter, demarcated from the dissoconch. It may belong to a new species of Brachidontes, but the adult size is not clearly recognized because the shell-size of these specimens is quite variable.