Repository.—All the specimens described in this article are preserved in the University Museum, University of Tokyo (UMUT). Each register number represents all the specimens of a species collected from one and the same cave, and type and illustrated specimens are specified with a suffix (a, b, c,…). Most living specimens, except illustrated ones (dried for SEM photography), are preserved in alcohol for further studies.

Abbreviations.—The following abbreviations are used in the description; RV: right valve, LV: left valve, CV: conjoined valves, Pd I: prodissoconch I, Pd II: prodissoconch II.

Illustrations.—All the figures except for the illustrations of a few large-sized species are SEM photomicrographs unless otherwise stated.

Measurements.—Measurements (Length × Height, accuracy: 1/20 mm) or magnifications of illustrated specimens are given in figure captions. Prodissoconch size was more accurately measured under SEM, and its range and mean are given in the description and Table 4.

Trivial names.—New Japanese trivial names are given in the figure captions.