There were several characteristic thin-shelled extinct bivalve genera, such as Daonella, Halobia, Monotis, Bositra, Buchia and Inoceramus in Mesozoic times. Their worldwide geographic distribution and short stratigraphic range provide useful information for Mesozoic biostratigraphy. They are also noticeable for their peculiar morphology and unique mode of fossil occurrence. Furthermore, they are important for evolutionary studies because they show rapid "evolutionary rates". The paleobiogeographical significance of these bivalves has also increased with development of plate tectonic theory. Many problems concerning such aspects as classification, ecology and evolution, however, remain unsolved.

Among others, Monotis occurs in abundance in various areas of Japan except Hokkaido and seems to offer excellent material for evolutionary studies at the population level. Monotis is also found abundantly worldwide in Upper Triassic strata, exclusively in a part of the Norian or the Norian-Rhaetian, in which amnnonoids and other index fossils are comparatively rare. The absolute time range of Monotis is only a few to several million years, according to Harland et al. (1982) and Tozer (1980b). Therefore, Monotis has long been regarded as an important index fossil.

The classification of Monotis, both infraspecific and supraspecific, is now considerably confused owing to excessive splitting and typological treatment of specimens. More than 60 specific and infraspecific taxa have been proposed by various authors, as reviewed by Westermann (1973b) and Grant-Mackie (1978a). Keyserling (1848) described Monotis species as "Avicula Ochotica minor", "A. Ochotica media" and "A. Ochotica major" from the south coast of Okhotsk Bay, all of which are trinominal and invalid now. Teller (1886) described the three forms from Verkhoyansk in northeast Siberia as "Pseudomonotis" ochotica, disregarding Keyserling's second specific names, and proposed five additional varieties, namely "densistnata", "sparsicostata", "eurhachis", "ambigna" and "pachypleura". After then, many paleontologista have proposed and described several infraspecific taxa within this nominal species, and have added other closely related species. In Japan, for instance, Kobayashi (1935), Sakaguchi (1939), Kobayashi and Ichikawa (1949), Ichikawa (1951b), Bando (1961), Nakazawa (1963, 1964a) and Tamura (1965) described many species, subspecies and varieties, most of which were critically listed by Hayami (1975, pp. 68-71). Several "subspecies" of a single species or some closely related "species" were often reported from one and the same locality, Considering the modern concept of systematics, such coexistence of many subspecies or closely related species seems to be rather unlikely.

Hayami (1969, p. 378) criticized the hitherto proposed typological classification of Monotis species from Japan, and preferred a hypothesis that all the specimens of Monotis in a fossil bed generally belong to one and the same species. His monospecific hypothesis of Monotis faunas is here taken as an important premise, and I have attempted to test it by means of biometrical and other methods. The classification of Monotis, I believe, cannot be appropriately evaluated, unless intrapopulational variation is recognized. During the present study wide morphological variation has been often recognized within one and the same fossil population.

It is the primary purpose of the present paper to analyze Monotis morphology with fossil population samples from throughout Japan, and to solve the confusion of Monotis classification. The second purpose is to detect morphological changes of Monotis with time in Japan in order to reach a better understanding of Monotis phylogeny. Thorough investigation of evolutionary changes seems to indicate a chronocline. The third purpose is to interpret, if possible, how and where Monotis lived, especially on the basis of the mode of fossil occurrence and shell morphology. The mode of fossil occurrence may furnish valuable clues to the paleoecology of Monotis.