Abstract
A total of 68 specimens of thick-shelled, peculiarly shaped Miocene mytilids from Hokkaido and northern Honshu were examined from the viewpoint of constructional morphology. They have been treated as members of three species : Mytilus (Plicatomytilus) ksakurai, M. (Tumidimytilus) tichanovitchi, and M. (T.) furanuiensis, with different shell shapes, muscle scars and surface sculpture. M. ksakurai shows an abrupt increase in shell convexity during ontogeny, like that in M. tichanovitchi. It develops a few conspicuous plications just after this allometric change in shell convexity. Polished sections of the shell of M. ksakurai reveal that just after the abrupt change in shell convexity, the internal growth increments bend sharply toward the inside of the shell. This is associated with a remarkable thickening of the outer part of the fibrous prismatic layer of the shell. These facts suggest that in M. ksakurai, the mantle tumed inward at the stage of the allometric change in shell convexity. Consequently, the diverging plications were formed by the wrinkled mantle as a result of the reduction in space across which the mantle extended.
