The classification of Japanese Monotis is fundamentally reconsidered from the viewpoint of population paleontology instead of typological and purely morphological treatment. Four species, namely M. scutiformis, M. ochotica, M. sabaikalica and M. mabara are distinguished, and the second species is separable into two chronological subspecies, namely M. ochotica densistriata and M. ochotica ochotica. Because these species, except M. mabara, seem to constitute almost a single evolutionary lineage, the limit of each taxon is rather arbitrary, and various intermediate fossil populations may exist. The main diagnostic characters of each species and subspecies are as follows:
M. scutiformis (Teller, 1886): 25 to 40 fine primary plicae with frequently inserted higher-order ones.
M. ochotica densistriata (Teller, 1886); 15 to 30 fine and sharp primaries with regularly inserted and somewhat weaker secondaries, and scarce tertiaries.
M. ochotica ochotica: 8 to 25 primaries of variable strength with irregularly inserted secondaries and tertiaries.
M. mabara: 15 to 30 fine and equal-sized primaries almost without higher-order plicae.
M. zabaikalica: radial plicae disappear after the middle growth stage.

Monotis-bearing formations are distributed in 22 areas in Japan. The most complete and typical section is found in the Utatsu area of the southern Kitakami Mountains, where M. scutiformis, M. ochotica densistriata, M, ochotica ochotica and M. sabaikalica (plus M. mabara) occur in upward sequence. The successive occurrence of these forms, except M. mabara, constitutes four Monotis zones. M. mabara is found very rarely with M, zdbaikalica. In spite of incompleteness, all the Monotis sequences in various areas of Japan arc consistent with that of the standard section.

As compared with other ordinary Mesozoic bivalves, Monotis has an extraordinarily gregarious and exclusive occurrence. The modes of fossil occurrence are classified into 12 types in terms of such criteria as rock type, lamination, shell abundance and shell fragmentation. Japanese Monotis beds are generally characterized by coarse-grained clastic sediments. The sedimentary fades suggests a shallow-sea environment for the Monotis-bearing formations, even if the Monotis beds are found in slump deposits such as breccia or blocks. In these respects the habitat of the Japanese species of Monotis must have been considerably different from that of M. salinaria which occurs in the pelagic micritic limestone fades of the Tethyan province.

At present, the ecology of Monotis must be inferred from such indirect evidence mode of occurrence and functional morphology. The presence of an anterior auricle, byssal notch and pseudoctenolia suggests that Monotis attached itself by a delicate byssus to such objects as seaweed and gorgonacean anthozoa. Because the occurrence of Monotis, at least in Japan, is restricted to near-shore and shallow-sea sediments, these Japanese species may have been benthic. However, this may not be true of M. salinaria which was possibly pseudoplanktonic, as inferred by some authors.

Shell morphology, as well as intra- and inter-populational variation, were studied based on Japanese materials. Among the various morphological characters, the structure of the byssal notch in the right valve may be important in considering the taxonomic position and ecology of Monotis. Several well preserved specimens of M. ochotica densis-triata reveal the structure of pseudoctenolia on the upper and lower sides of the byssal notch. The presence of this character seems to support Waller's (1978) system in which the Monotidae was included in the Buchiacea, and also suggests a byssate mode of life for Monotis.

Extremely wide morphological variation, especially the surface ornamentation and the development of a posterior wing, exists in every population sample of Monotis. The intrapopulational variation is generally continuous, as exemplified by the normal distribution of such a meristic character as the number of plicae, The result of bio-metrical analysis also seems to justify the hypothesis that all the specimens of Monotis in a fossil bed probably belonged to a single species.

Most of Japanese species of Monotis are taxonomically identical with the species from east Siberia (Verkhoyansk and Kolyma Mountains), and inclusion of both regions in the same faunal province at that time is strongly suggested. Differences in Monotis faunas among the major structural divisions of the Japanese Islands are not significant, though the Sambosan Belt of the Outer Zone yields quite different Triassic bivalve faunas which bear some obvious Tethyan elements.

In Norian time there are several other provinces characterized by particular species group(s) of Monotis; for instance, the Tethyan province by the M. salinaria Group, and the Maorian (New Zealand and New Caledonia) province by the M. calvata Group. Some disjunct distribution of Monotis species may be explicable by assuming movement and accretion of "suspect terranes" but this may be an untestable hypothesis.

Among various Mesozoic bivalves, Monotis is an exceptional genus for its rapid morphological change and the presence of plentiful material at numerous localities. Though the intrapopulational variation is generally wide, morphological change with time can be detected by a biometrical treatment.

It is presumed that phyletic evolution occurred in the lineage from M. scutiformis to M. ochotica ochotica through M. ochotica densistriata, as is exemplified by Shirayama-zawa and other sections. This lineage shows a gradual decrease of the number of primary radial plicae, an evolutionary trend which can be regarded as paedomorphic. Two distinct lineages seem to have originated from the populations of M. ochotica ochotica. One is M. zabaikalica, which was probably in direct descent from M. ochotica ochotica and is characterized by the accelerated (or peramorphic) disappearance of radial plicae. The other species, M. mabara appears to have been derived from the main stock of M. ochotica-M. sabaikalica. This branch exhibits an opposite trend with an increase in the number of radial plicae.

In this study, the mechanism causing evolutionary change described above would not be recognizable due to the incompleteness of the fossil record. However the evolutionary changes of Japanese Monotis appear to be phyletic evolution along a single lineage. The M. ochotica-M. sabaikalica sequence does not support the stasis maintained in punctuated equilibrium theory. If, in fact, these evolutionary transitions occurred during only a few million years, the evolutionary rate of Monotis must have been extremely high.

Monotis seems to be an interesting fossil organism because of its wide geographic distribution, gregarious and exclusive occurrence, and rapid morphological change. The classification, stratigraphic distribution and mode of occurrence studied here were based on Japanese materials, but this is only the first step in a comprehensive study of Monotis, because the evolution of a taxonomic group should be clarified by evidence from other parts of the world. Four major biogeographic regions can be recognized: East Asia (east Siberia and Japan), Tethys (Alps, Himalayas, etc.), western rim of Americas (Alaska, British Columbia, Nevada, Peru and Chile) and Maorian (New Zealand and New Caledonia). Each region is characterized by one or a few species groups of Monotis, but the phylogenetic relation among the groups and their ecology must be further explored by more extensive comparative studies.