Abstract
Although it is needless to say that the radula of gastropods is one of the important characters for classification, and we are favoured by abundant knowledge on its morphology, no literature dealing with the sexual dimorphism in the radula has been publishod, so far as I am aware. In this paper, it is cleared up that Drupella shows a remarkable sexual dimorphism in its radula. It has already been pointed out by some predecessors that Drupella is a peculiar group in having quite aberrant radulae with slender reed-shaped laterals and with various forms of rachidians, unlike those of all other species of Muricidae. D. cornus (Roding) and D. fragum (Blainville) described here, show a conspicuous difference between male and female, in forms of rachidians, in ratio of lateral rows to rachidians, in proportion of length of lateral to breadth of rachidian, as well as in color of radula etc. as summarized in Table 3. Cooke (1895) and Thiele (1918) gave each a different figure concerning the radula of D. spectrum (Reeve). Juding from the result of my observation, it is inferred that these discrepancies of radula may be due to the sexual dimorphism, and that Thiele's figure the radula of male. by having a large, thick rachidian, while Cooke's one that of female, by having a triangular rachidian with needlelike inner denticles. D. fragum (Blainville) has been placed in Drupa by previous authors, but it should be transferred to Drupella, because the characteristic features of its radula show no sign of those of Drupa but those of Drupella as shown in Pl.6.
