| Summary: | Twenty six species of bivalve belongins to fifteen families are recorded fron muddy bottom at bathyal zone (996−1948m) off Kochi Prefecture by beam trawl survey during two cruises of R. V. Tansei−Maru. Tindaria sp., Vesicomya indica, Pholadomya sp. are first records of Japan. Most of the species collected are endemic in the intermediate water of bathyal zoon of Boso-Peninsula･Sgami-bay and Kii Peninsula. Of the species collected in this surbay, three species, Acharax johnsoni, Cyclopecten bistriatus, Abra profundorum are arctic in origin and seven species, Propeamussium watosoni, Delectopecten alcocki, Limatula sibogai, Vesicomya indica, Pholadomya sp., Cuspidaria mitis are tropical in origin. Molluscan fauna in the bottom at 1000m and 1500-2000m depths between are different; the bottom of the former is muddy flat and the later is sharply sloped and mud mingled with pumices. It seems that there is their transitional zone between 1300m and 1500m depth.
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