Abstract: This paper deals with ostracod fauna of the northern part of the Boso Peninsula. These ostracods lived in the Paleo-Tokyo. Bay during four periods of high sea level accompanying interglacial stages (ca.300,000-100,000 yr B. P.), as represented by the deposition of the Yabu, Kiyokawa, Kamiiwahashi, and Kioroshi Formations, which are the middle and upper parts of the Shimosa Group. Each formation shows a cyclic tendency reflecting a cycle of glacioeustatic sea-level changes. The Paleo-Tokyo Bay may have been open toward the east, however, during the maximum high sea-level phase, warm water could have entered the bay directly from the south.

Ostracods representing 121 species were identified. Most of the modern species of ostracods living in the shallow sea of the temperate zone on the Pacific coast of Japan are represented. Four assemblages were recognized: subtidal sand, warm water sand, shallow water mud, and brackish water clay. Warm and cold water influences are represented by the displacement of several species within one assemblage.

One genus, Robustaurila, and 14 species are described as new: Neocytherideis aoi, Eucythere yugao, Munseyella oborozukiyo, Callistocythere hotaru, Schizocythere asagao, Campylocythereis? ukifune, Aurila kiritsubo, Pseudocythere frydli, Eucytehrura utsusemi, Semicytherura wakamurasaki, Kangarina hayamii, Loxoconcha hanachirusato, L. tamdkazura, and Xestoleberis suetsumuhana. The following genera are reported from Japan for the first time: Paracypris (Candonidae), Eucythere (Eucytheridae), Stigmatocythere, Rocaleberis, Carinovalva, Robertsonites, Anstralimoosella, Campylocythereis (Trachyleberididae), Pseudocythere(Bythocytheridae), and Kangarma (Cytheruridae).