1) There has been recognized through the observation both of surface and in profile little difference in the morphology of the ligamental grooves and ridges, but the peculiarity to each species occur only in the breadth of grooves.
2) The rate of increase in number of ligamental ridges is nearly parallel to the age of the shells. and consequently it can be regarded as an indicator of the growth stages.
3) The specimen-groups of
Glycymeris yessoensis from various localities and horizons have their own growthlimit of shell size, and the numbers of ligamental ridges are almost equal near the growth limit.
4) The growth curves of several specimen-groups of the same species from various localities appear to have branched off from a common trunk, and therefore. concerning the self-accelerating phase of shell-development, the relation between the shell-size and the number of ligamental ridges is peculiar to the species.
5) It is recognized that the more rapid the shell-size increase, the smaller the shell-convexity among either the same species or different species.
6) The large size of the “Sasage specimen” is judged to have been resulted not from longevity, but from the much more rapid rate of increase in shell-sizes which seems to be consistent with their striking thickness.
7) For lack of specimens, the abovestated results could not be compared with the Recent ones, but a comparison with them from various localities or environments will serve as a basis for considering their ecological meaning.
View full abstract