The development of descriptive studies on Mesozoic Bivalvia from Japan and its adjacent areas, as enumerated in Table 2, reached its climax in the late 1950's and the early 1960's. Although some faunules, especially Upper Cretaceous ones, still remain undescribed, the majority of common species seem to have been described and named. I dare to say that we have almost completed the first stage of the systematic research. This stage ought to be followed by comparative studies for phylogenetic classification from international viewpoint and furthermore by detailed evolutionary studies at the population level, as usual in the development of systematics. Mayr (1963) rightly pointed out that the replacement of typological thinking by population thinking is one of the greatest conceptual revolution in the history of systematic zoology. It is so with the classification of fossils.

Among various fossil invertebrates the Bivalvia are often advantageous for the study of population systematics owing to the abundant occurrence, relatively simple morpho logical structure, convenient size for mensuration and a great store of knowledge about the ecology and physiology of living species. Indeed, as concisely reviewed by Newell and Kauffman (1968), many outstanding studies on fossil bivalves seem to have much contributed to the spread of population concept and related techniques in paleontology.

Most of Mesozoic bivalves from Japan had been described and named merely on the basis of individual morphology, before population systematics became popular among the paleontologists. As the present revision progressed, it was often felt that early described species were too much oversplit. On the other hand, systematic ordering of fossil Bivalvia as well as diagostic characters of all the genera and subgenera was comprehensively accomplished with the monumental publication of "Genera of the Bivalvia: a systematic and bibliographic catalogue" (Vokes, 1967) and "Treatise on invertebrate paleontology, Part N" (Cox et al., 1969, 1971), which provided great facility for a revisory work of this sort. I feel, therefore, that there is much room for improvement as the classification of these Japanese material, convincing that now is the convenient time to make a revised catalogue.

Owing to poorly preserved material and other fatal limitations in fossils, however, I often found great difficulty in obtaining solid taxonomic conclusions. Early proposals of new species were commonly not accompanied by any clear designation of type specimens, and exact localities and horizons of syntype specimens were rarely precisely recorded. In many cases topotype specimens, even if the locality is more or less clear, are not easily collectable. Quantitative studies on intrapopulational and geographic variations can hardly be pursued owing to strong secondary deformation and too small sample size except for a few special cases.

Nevertheless, the switch-over of the species concept from typological to biological in recent years, I believe, enables us to improve the classification at the species level. Many early proposed species and varieties can be reexamined in the light of modern taxonomic principles and nomenclature, taking variation, ontogeny, geographic and stratigraphic distribution and mode of occurrence as well as morphology into consideration. In revising taxonomic names of specific and infraspecific taxa, I attach importance to the relation of sympatry and allopatry as one of non-morphological criteria. Population biologists generally share the opinion that two populations cannot live sympatrically unless they are reproductively isolated. Although post-mortem transportation may disturb the original assemblage of species to some extent, this agreement should be taken into consideration also in the classification of fossils. For instance, we may well wonder whether taxonomic distinction is appropriate, when many closely related "taxa" were reported to occur from a single fossil bed. Although the revision can by no means be made automatically, more than 40 specific names are here regarded as junior synonyms, since oversplitting is obvious. More than 60 varietal names have been used for the classification of Japanese Mesozoic Bivalvia (mostly Triassic or Cretaceous forms). All of them were proposed before 1961 and, therefore, are available as taxonomic names of species-group according to the International Code of Zoological Nomenclature (Article 17). Some recent authors have automatically regarde them as subspecies. Examining the type localities and distribution of these varieties, however, these names were commonly given to some extreme specimens derived from the same fossil population. Apparent range of variation in shape often seems to be exaggerated by ontogenetical development and secondary deformation. In such cases taxonomic distinction appears to be biologically meaningless, and the varietal names should be regarded as junior synonyms. Only in a few cases when geographic or chronological segregation is assumed, the "varieties" may be regarded as representing subspecies or distinct species.

In the present study I regard many proposed specific and infraspecific names as invalid in accordance with the above-mentioned principle. It must be, however, emphasized that such taxonomic decisions are possible only in the cases when the original specimens are well preserved or were well described with necessary information about the occurrence. Most of the early collections had poor locality data, and other criteria than morphology are often hardly applicable. Taxonomic decision is especially difficult, when a taxon is represented only by one or a few poorly preserved specimens. There fore, there are many ambiguous cases as to the validity of hitherto proposed taxonomic names. As suggested by many authors, some Mesozoic bivalves from Japan are hardly distinguishable from the species proposed on the Tethyan, European and American materials, even if slight morphological or chronological differences may exist. Specific discrimination or identification between the materials from such distant regions often cannot be concluded on a firm basis, because of the different state of preservation and the deficiency of reliable information about the faunas from the midway areas. In many doubtful cases I was compelled to follow tentatively the classification adopted in previous works, but remarks on the possibility of different classification, if any, are added.

As to the classification of subgeneric and higher taxa I intend to take the middle path, avoiding both extreme lumping and splitting. The systematic ordering adopted in the present revision is nearly the same as that of The Treatise, but necessary amendments are presented about the taxonomic position and ranking of some genera and subgenera, if they have been misinterpreted or regarded as ambiguous.