Upper Pleistocene sediments of the northern part of the Boso Peninsula consist of dominantly marine strata which reflect cyclic changes of the sedimentary environment and contain well-traceable marker-tephras and well-preserved fossils. They represent the last stage of the filling-up of the middle and southern parts of the Paleo- Tokyo Bay, which opened toward the east and, at the time of high sea level phase, perhaps also toward the south. The sediments preserve eventful records of faunal changes resulting from sea level fluctuations during glacial and interglacial periods. In this paper, ostracods which lived during four interglacial stages of high sea level represented by the deposition of Yabu, Kiyokawa, Kamiiwahashi, and Kioroshi Formations are described. These formations correspond to the middle and upper parts of the Shimosa Group.

The geology and paleontology of this area has been studied intensively since the latter half of the last century. From the paleontological point of view, the following three types of studies are of special interest: (1) reports of fossil occurrence at the several famous localities, Ikebe (1936), Oinomikado (1937), Oyama and Ishiyama (1968), Ohara (1968), and Horiguchi and Ohara (1972); (2) discussions on changes of paleoenvironments based on areal geological surveys, Kozima (1958-1966) and Aoki et al. (1962-1971); (3) tephrochronological studies to find the exact interrelation between local depositional sequences, Sugihara (1970) and Sugihara et al. (1978).

Recently, ostracods from the Kisarazu area (A of text-fig. 1) were discussed in relation to the paleonenvironment reconstructed from detailed field observations (Yajima, 1978) of a reconnaissance study on the Japanese Upper Pleistocene warm-water ostracod fauna.

As the first step of the present study, the stratigraphy of the ostracod-bearing horizons was described to restore the physiographic environments of deposition. Data were obtained through field mapping and observation of sediments with special reference to the occurrence of megafossils. Each formation shows a cyclic tendency of sedimentation reflecting a cycle of glacioeustatic sea level changes.

As the second step, effort has been made to update the taxonomy of the Pleistocene ostracods of the Yabu to Kioroshi Formations. In many species, classification at the specific level remains in confusion. Examples can be seen in the species of Aurila and Loxoconcha. For a complete solution of the problem, the study of live material is often more effective than that of Pleistocene fossils. Published and unpublished information on the modern species was referred to as much as possible for species identification.

Several hundred new genera of ostracods have been proposed since the publication of the Treatise in 1961. Most of these are cytheracean genera and some attempts have been made to classify this large number of cytheracean taxon (Hartmann and Puri, 1974; Liebau, 1975; Gründel, 1973, 1976). Such attempts of classification were to a large extent successful, but many controversial questions still remain. For Japanese ostracods, many species of the Trachyleberididae and Hemicytheridae have still been assigned to inappropriate genera. This paper attempts to clarify this confusion and to classify Japanese trachyleberidids and hemicytherids into the proper genera.

Grouping of genera into tribes, especially into the two families mentioned above, raises another problem. Tribe by tribe or subfamily by subfamily revisions are now becoming popular among ostracod taxonomists. However, appropriate tribes to accommodate some genera which occur in Japan have not yet been found. Although hasty assignment may result in confusion, it is not practical to wait till satisfactory tribeby-tribe classification is available. Thus, the resultant family group classification used in this paper is quite tentative.

As the third step, the Pleistocene ostracods of the area were analyzed ecologically. The Pleistocene ostracods described in this paper seem to include most of the modern species of ostracods living in the shallow sea of the temperate zone on the Pacific coast of Japan. Therefore, the information on the ecology of modern species found in the checklist of the Japanese ostracods (Hanai et al., 1977), ecological study of ostracods of Hamana-ko Estuary (Ikeya and Hanai, 1980 MS) and unpublished data on ostracod ecology of Aburatsubo Cove, Misaki, was especially useful in the analysis. Time and space distribution of the Pleistocene ostracods of this area may be explained by interfingering of ostracod fauna of warm-temperate and cold-temperate water in this area. Four assemblages are recognizable; subtidal sand, warm water sand, shallow water mud, and brackish water clay. Warm and cold water influences may be represented by the displacement of certain species within one assemblage.