Taxonomical, paleoecological and biostratigraphical studies of the middle Miocene (Helvetian) fishes from the western area of Shizukuishi basin in Iwate Prefecture, North-eastern Japan, are presented in this article. The fossil bearing formations are Kunimitoge and Koshitomaezawa formations. The former is correlated to the Oishi formation (KITAMURA, N., 1959, 1960) and yields the following warm mesopelagic fishes : Bathylagidae ; Bathylagus sencta, sp. nov., B. obesa sp. nov., Genostomatidae; Ohuus hitamurai gen. and sp. nov., Myctophidae; Diaphus shizuhuishiensis sp. nov., D. sp. (1), D. muraii sp. nov., Lampadena nanae sp. nov., Brotulidae; Glyptophidium litheus sp. nov., Moridae; Lepidion miocenica sp. nov., Coryphaenoidid and Cottid fishes. The reason of mass mortality of these fishes may be attributed to the submarine volcanic activity. The fossil fishes found in the latter are Diaphus sp. (2) and Itatius (?) sp. (Family Brotulidae), which appear to associate with the heteropic benthos of socalled "Yama fauna".

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The Paleo-Tokyo river has been known by the topographic feature of the Gulf of Tokyo, which formed probably at the Warm regressions. The sonic prospecting was carried out here and property of river is revealed. At the northern part of the Gulf of Tokyo, it is recorded as the sonic gravel layers where the sounds are scattered by obstacles under sea bottom. At the southern part, the river channels are recorded which are formed by engraved Neogene Tertiary system and buried with Quartanary sediments at transgressions. The sea bottom slope along the river is about 1.7-2.6 milli-radian. It is rather steep compared with rivers in the plane. Surely the northern part of the river is shallow and the southern part is deep. The slope of river bed of channel is inverse, and is about 1.0 m-rad. It is considered that it flows from north to south, and after that the basement Neogene Tertiary has moved to tilt inversely. There are terraces above river beds, and it is recognized by the records that the river beds (bottom of channel) are older and terraces are younger. It is guessed the river was formed at the first and last regression of Warm. At Yokohama shore off the Paleo-Tokyo river judged by the sonic gravel layer separates from topographic river and comes to the vicinity of the Tsurumi river. It coincides with geologic data about the river beds of the Paleo-Tama river. There remain some questions of relations between the river and the Tokyo Submarine Canyon.

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