Parastegodon is a valid genus, though it little differs from the genus Stegodon, and specialization is continuous from one to another. Japanese species of Parastegodon are closely related to the Malayan forms of the so-called Stegodon (mindanensis, trigonocephalus and airawana); the latter like the former are distinct from the genotype of Stegodon. Parastegodon contains the following species; P. aurorae (MATSUMOTO), P. mindanensis (NAUMANN) MATSUMOTO, P. trigonocephalus (MARTIN) SHIKAMA, P. airawana (MARTIN) SHIKAMA, P. akashiensis TAKAI, P. infrequens SHIKAMA and P. kwantoensis TOKUNAGA. From dental and cranological characters, Parastegodon is classified into three groups of airawana, akashiensis-aurorae and infrequens-kwantoensis. It is an oriental stock, having intermediate characters between Stegodon and Archidiskodon. In Japan it seems to have dissappeared at the end of the Tertiary owing to its catagenetic declination.
Surface of the shell of Crassatellites foveolatus (Sow.) is ornamented only with. concentric ribs; beneath this surface layer, there is a layer which is characterized by radial ribs. The existence of this remarkable feature has been found recently in the course of studies with the recent and fossil specimens collected in and around Taiwan, as is shown by the figures 5, 6 and 7 on the annexed plate. When the surface, concentrically ribbed layer is worn off, it gives rise to a feature shown by fig. 4 looking as if to belong to a different species. On examining under the microscope, thin sections, both transverse and longitudinal, of this and many other kindred species, the author has become confirmed of the fact that some of the shells with crenulated internal margin have a sub-surficial, radially ribbed layer, although in no other case the feature has been so conspicuous as in the species under consideration. Within certain limits, such a peculiar feature of shells may play an important part in the specific or even generic identification among pelecypods.