As briefly reviewed above, studies on the pre-Tertiary gastropods from Japan and its adjacent areas still remain no more than descriptions of local fauna. Little investigation has been made of the paleoecology and phylogeny. Taxonomic criteria have almost been restricted to the shell morphology at the individual level, and little attention has been paid to the intra- and interpopulational variation, stratigraphical and geographic distri-bution and mode of occurrence. In the case of Mesozoic bivalves, one of us (I. H.) was worried about the difficulty in allowing variously split nominal species to come near real ones. Much deficiency of taxonomic data was also noted in Paleozoic and Mesozoic gastropods during the preparation of this volume. However, descriptive works on the present group are quite sporadic, and oversplitting of species is not so conspicuous, except in a few cases. Most of hitherto proposed species of Paleozoic and Mesozoic gastropods, though they still remain only nominal, seem to be valid at least nomencla-turally. Therefore, only a few changes are made here as to the specific discrimination and identification. On the other hand, with hitherto proposed infraspecific names (i.e., subspecies, varieties and formae), we have adjusted them as far as possible in accordance with the International Code of Zoological Nomenclature.

In preparing the present taxonomic catalogue, we faced two more difficult problems. One was the generic and subgeneric assignment of the gastropod species; they are often not typical of hitherto established genera and subgenera, which were mainly based on European and American species. Consequently, we were often driven to add a question mark after a generic (or subgeneric) name, even if the material is well preserved and was precisely described. As the taxonomic studies on this group progress in the future, a considerable number of new generic or subgeneric names will be required; in the present catalogue no attempt is made to provide such new names.

The other problem is attributable to the unsettled major subdivisions of this class. Various classification systems of gastropods have been presented by many authorities (e.g., Pelseneer, 1906; Thiele, 1929-1931; Wenz, 1938-1944; Pcelincev et al., 1960; Knight et al., 1960; Taylor and Sohl, 1962; Golikov and Starobogatov, 1975), primarily for the Prosobranchia. They often differ considerably, not only in the names of higher taxa but also in the taxobasis and contents. It is often difficult to determine the taxonomic position of Paleozoic and Mesozoic extinct groups (e.g., Murchisoniacea and Nerineacea). Moreover, homoeomorphism does not seem to be rare.

One of the standard and reliable classification systems is found in the Treatise on In vertebrate Paleontology, Part I (Knight et al., 1960), in which all the hitherto described genera and subgenera are taxonomically arranged. Yet, this volume treats only the taxa belonging to the Archaeogastropoda and Paleozoic and a few Mesozoic groups of the Caenogastropoda and Opisthobranchia. The volume (Part J) treating most families of the Caenogastropoda and Opisthobranchia has not been published. Very recently Golikov and Starobogatov (1975) have presented a new but somewhat controversial classification of the Prosobranchia which proposes many higher taxa. Since we are not in a position to present a more adequate scheme of major classification, the systematic ordering adopted in the present catalogue is mainly in accordance with that of Knight et al. (1960) for the Archaeogastropoda and with that of Zilch (1959-1960) for the Opisthobranchia and Pulmonata. For the classification of the Caenogastropoda we owe much to the comprehensive survey of Wenz (1938-1944), but a few rearrangements are made here on the basis of recent taxonomic studies. For example, the Heterogastropoda (Epitoniacea and Architectonicacea) are separated from the Mesogastropoda as a distinct order, and the Nerineacea are regarded as belonging to the Opisthobranchia.

Palaeacmaea and some other Lower Paleozoic patelliform genera and Hyolithes are now generally excluded from the Gastropoda and regarded as constituting distinct classes, the Monoplacophora and the Hyolitha, respectively. Some representative species from the Cambro-Ordovician of Korea are listed in the present catalogue, since they were originally described as gastropods. It has been suggested in some recent studies that the suborder Bellerophontina wholly or partly belongs to the class Monoplacophora. In the present catalogue, however, the representatives of this suborder are listed as gastropods, since more evidence seems to be needed.