Two more trochoid species are added to the gastropod fauna in North Knoll of Iheya Ridge, Okinawa Trough, based on new collection of Shinkai 2000 : Iheyaspira lequios n. gen. et n. sp. characterized by skeneid-like shell and 12 lateral teeth of radula, and Margarites ryukyuensis n. sp.

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A single new subgenus, Perspicioscala and 14 new species, Cycloscala spinosa, Kurodacira glans, Cirsotrema (Cirsotrema) amamiense, Cirsotrema (Elegantiscala) amplsum, Gyroscala (Circuloscala) watanabei, Amaea (Narvaliscala) kushimotensis, Claviscala nodulosa, Claviscala nagaii, Claviscala subulae, Epitonium (Parviscala) tenuipicturatum, Epitonium (Perspicioscala) ossium, Epitonium (Perspicioscala) nodai, Epitonium (Perspicioscala) kiyohimae and Alora kiiensis are descrided. Three substituted names, Amaea (Narvaliscala) percancellata, Epitonium (Epitonium) profundum and Epitonium (Parviscala) graciliconfusum are proposed for subcancellata, okezoko and confusa, respectively. These species are figured and described in detail.

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Pterynotus (Pterynotus) fernandezi n. sp. is described from Sea Mount Vema, 600 km off Cape Town, South Africa. It is compared with P. phaneus (Dall, 1889) and P. havanensis Vokes, 1970 from the western Atlantic, P. atlantideus Bouchet & Waren, 1985 and P. (?P.) leucas (Fischer, 1897) from the eastern Atlantic, and P. fulgens Houart, 1988 from New Caledonia and South Africa.

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Geometric features of aggregated biomineral units were examined in a total of 13 species of the Bivalvia both theoretically and empirically. Fibrous prisms and foliated blades on the external shell surface and composite prisms in shell section parallel to the outer shell surface were studied. As a result of SEM observation, it was clarified that microstructural units tend to be slightly oblique with respect to growth lines in the ventral direction near the anterior or posterior shell margin. The angle between the biomineral unit and growth lines varies within a single valve. These geometric features of the microstructures are well reproduced by computer simulation when the growth rate of the structural unit in the radial direction is larger than that in the transverse direction.

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The bacterial endosymbionts of the vesicomyid clam Calyptogena fossajaponica, collected from the hadal zone in the Japan Trench, were characterized. Transmission electron microscopic (TEM) observations showed numerous bacteria in epithelial cells of the gill tissues of C. fossajaponica. Sulfur content and isotope analyses of this clam also suggested the presence of sulfur-oxidizing bacteria (thioautotrophs). Phylogenetic analysis based on the 16S ribosomal RNA gene (rDNA) sequence indicated that the bacteria was related to sulfur-oxidizing endosymbionts of other vesicomyid clams from deep-sea hydrothermal vents and cold seeps. The symbionts of C. fossajaponica formed a monophyletic group with the symbionts of C. magnifica, C. phaseoliformis and Calyptogena sp. (Florida Escarpment). These 4 species live relatively deeper than other vesicomyid clams whose symbionts have been phylogenetically characterized. Vertical distribution patterns of vesicomyid clams are likely influenced by their symbionts.

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The Nakazato and the Tobiyozaki sand dunes, located on the Kikaijima Island in the Amami Archipelago, intercalate a layer of paleosol. Each paleosol layer was formed at the same time 2, 000-3, 000 yrs ago. The Nakazato land molluscan fossil assemblage, which occurs in the paleosol layer of the Nakazato sand dune, consists of 15 species of fossilized land molluscs. The Tobiyozaki land molluscan fossil assemblage in the Tobiyozaki sand dune consists of 13 species. While minute fossils occurred abundantly, large fossils are few in number but have remained intact. This indicates that these fossil assemblages are autochthonous. Each fossil assemblage does not include four extinct species of land molluscs which occur in three paleosol layers of the late Pleistocene Suitengu sand dune on the Kikaijima Island. They became extinct from 27, 000 to 2, 000 yrs ago. In particular, Cochlicopa lubrica and Pupilla (Gibbulinopsis) cryptodon, are now distributed chiefly in comparatively cool regions, such as the Japanese mainland, the Chinese Continent and the Korean Peninsula. These two species are considered to have been extinct in the postglacial hypsithermal interval. In contrast, each fossil assemblage includes Allopeas pygula, Allopeas clavulium kyotoense and Succinea sp., which are not found in the assemblage of the late Pleistocene. They may have been carried by human activities. The Nakazato land molluscan fossil assemblage is composed largely of Gastrocopta (Sinalubinula) armigerella and Tornatellides boeningi, which now inhabit near the coast lines. The Tobiyozaki land molluscan fossil assemblage is composed largely of Georissa japonica and Nakadaella micron, which now inhabit in woodland. Therefore, it is inferred that the Nakazato land molluscan fossil assemblage was formed near the coast lines and that the Tobiyozaki assemblage was formed in and around the forests.

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