From Japan and its adjacent areas (North and South Korea, Saghalin and Taiwan) approximately 80 Paleozoic and 110 Mesozoic gastropods have been described and named in more than 70 scientific papers prior to 1976 (Tables 1,2). This number is small in comparison with that of the bivalve species (771 Mesozoic and 66 Paleozoic species in more than 260 papers) from the contemporary strata of the same surveyed areas. Provided that the average longevity of species is nearly equal between gastropods and bivalves and that the specific diversities of the two classes are roughly proportional to those in modern seas, our knowledge about the pre-Tertiary gastropod species in this region remains less than 5 percent as complete as that about the bivalves. Investigation of Paleozoic and Mesozoic gastropods is lagging far behind that of bivalves.

This marked contrast, we presume, is primarily due to technical difficulties in obtaining the material for study rather than to differences in fossil records. Generally speaking, gastropods as well as other orgnisms have been well known from calcareous sediments. For example, in the Cambro-Ordovician limestones in Korea, the Permian Akasaka limestone in central Honshu, the calcareous sediments of the Lower Cretaceous Miyako Group in north Honshu and the calcareous concretions of the Upper Cretaceous groups in Hokkaido, many well-preserved gastropods have been described, and the number of known species therefrom sometimes surpasses that of the bivalves. In contrast, molluscan shells are rather difficult to remove from non-calcareous consolidated rocks, since the tests are, if preserved, much weaker and more fragile than the clastic matrix. In many cases fossils are represented by internal and external moulds. Taxonomic investigation of gastropods in such a state of preservation, particularly species of Small size, is much more difficult than investigation of bivalves, because the apertural and other important three-dimensional characters are hard to recognize. Although some recent attempts have proved that the artificial casting from natural moulds by means of latex or silicone rubber is quite effective for taxonomic studies, almost nothing has been done on the description ofgastropod faunas from the widely distributed non-calcareous strata in the Paleozoic and Mesozoic terrains of Japan. The numbers of described gastropod species in different periods (Table 2), which appear to be much biased, may also reflect this circumstance.