1. The growing direction of the left valve of the oyster changes upward at the projected or hollowed portion of substratum. This phenomenon is remarkable in the smaller species and the southern population of certain species and also in the species found on the upper portion of the intertidal zone. 2. Generally if the left valve bends upward, the right valve is flat. But, if the left valve is flat, the right valve becomes deep. The right valve of larviporous species is also of deep form when the left valve bends upward. 3. The morphological variation of the oyster shells parallels the variation in weight of the shell valves. Namely in the case of the upwardly bended left valve, the left valve is heavier than the right valve, and if the left valve is flat, the right valve is the heavier. In the species which have symmetrical valves as larviporous species, both the left and right valves are almost equal in weight. 4. The relation of the form and weight between the left and right valve is characteristic in each species. Namely the degree of the said variation became smaller in the order of Crossostrea echinata, C. gigas, C. nippona, C. rivularis, O. circumpicta and O. denselamellosa. 5. The shell variation of C. nippona is rather similar to that of C. gigas.
