Although attention has been paid to dimorhism in the shape of shell of Ficus subintermedia (D' ORBIGNY) by some conchologists (SHIKAMA, 1957), no worker has yet shown any adequate cause to what it is to be ascribable. In this study, to ascertain the cause of dimorphism, observations with structure and habits of the living animal, as well as biometric measurements of various parts of the shell, were made based upon a good number of specimens with soft parts collected mainly off Mikawa-isshiki, Aichi Prefecture. From the results obtained in our careful analysis of all the data on this subject, it may be suggested that the dimorphism is attributed to sexual, more than to ecological, genetic and other casues, as demonstrated in text-figs. 4, 5, 6 and 7, as well as in table 1.

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