A new species of mytilid mussel, Gigantidas horikoshii n. sp., is described. This new species was found burrowing in a sandy bottom from which warm water was seeping out, on the submarine volcano Kaikata Seamount, located southwest of the Ogasawara (Bonin) Islands in Southern Japan. G. horikoshii n. sp. is characterized by the absence of a welldeveloped valvular siphonal membrane, the absence of a continuation of the inner mantle fold across the anterior adductor muscle and by the attachment point of the anterior retractor on the valve being situated out of the umbonal cavity in the same manner as in G. gladius, the only other known species of the genus Gigantidas. The present species is distinguishable from G. gladius by the large swollen umbonal cavity, the markedly concave ventral margin and shell proportions such as shell height and shell width relative to shell length.

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A new species of the Chamidae from Japan is described under the name of Chama cerinorhodon n. sp. Chama cerinorhodon has hitherto been misidentified as Chama fragum Reeve, 1846, from its superficial morphological characters. However, it is here revealed that this species is sufficiently distinct from true C. fragum in shell mineralogy and shell structure to warrant separate status. Chama cerinorhodon has a calcitic outermost shell layer in addition to aragonitic inner and median shell layers, whereas C. fragum has only aragonitic inner and outer shell layers. Chama cerinorhodon is closely related to the eastern Pacific species Chama arcana Bernard, 1976, and Chama pellucida Broderip, 1835, in shell morphology, mineralogy, and structure.

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Calyptogena (Archivesica) garuda n. sp. is described from the Java Trench at 2064-2137 m. The shell is large and solid, and attains 24 cm in length. The ventral tooth of the right valve is very strong, recalling a canine tooth. The subumbonal tooth of the left valve is fused dorsally. The pallial sinus is deep and acute.

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As a result of field investigation of the limpet fauna in southwestern Java, Indonesia, we have found 9 species of limpets (belonging to 3 families and 4 genera) including a new species. Lottia luchuana (Pilsbry, 1901) and Cellana toreuma (Reeve, 1854) are new records from this area. A new species of the Patelloida profunda group is described in this paper. Members of this group are morphologically similar to each other, but Patelloida javanica n. sp. can be distinguished by the orange colour of the intermediate area between the muscle scar and the shell margin, and by its vague yellow or reddish brown radial streaks.

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Two new species of alycaedid gastropods collected from Miyazaki Prefecture, Southeastern Kyushu, Japan, are described: Chamalycaeus shiibaensis and C. nishii. The former species is morphologically most similar to C. sadoensis (Pilsbry & Hirase, 1903), from which it is distinguishable in having fine ribs behind the aperture and by the shape of the unreflected peristome. The latter species resembles C. takahashii Habe, 1976 and C. tsushimanus (Pilsbry & Hirase, 1909), but differs from them by its smaller size and a swelling on the neck.

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The genetic differentiations between seep areas off central Honshu (the Japanese mainland) and hydrothermal vent fields in the Okinawa Trough were examined for two vesicomyid species, Calyptogena okutanii and Calyptogena nankaiensis, on the basis of nucleotide sequences of the mitochondrial gene for cytochrome c oxidase subunit I. For each species, significant genetic differentiation was detected between the two sea areas. Relatively highly diverged haplotypes were detected for both species. An asymmetrical distributional pattern of haplotypes was also shown for C. okutanii. As a similar phylogeographic pattern was revealed for another species inhabiting both sea areas, namely, C. solidissima, the migration of three Calyptogena species between seep areas off central Honshu and hydrothermal vent fields in the Okinawa Trough might be attributable to the same historical events.

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Genetic and morphological divergence in the endangered freshwater pearl mussel Margaritifera laevis from Nagano Prefecture is investigated. We reveal the existence of two genetically divergent forms within the mussels. The two forms occurred sympatrically in one of four sampling locations and displayed complete allelic substitutions at four allozyme loci, suggesting an absence of hybridization between them. Their shell morphometries were also different. These findings strongly suggest they are two distinct species. A conservation program should be designed for each of the two forms.

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Host preference in Margaritifera laevis was investigated via field and laboratory studies on two populations in the Shinano River System. Although there was no difference in the growth curve of glochidia during the parasitic period, or in the size of the resulting juvenile mussels, the host species were different. The masu trout (Oncorhynchus masou masou) was the host in the Chubu-Nogu River but not in the Sakasa River, whereas the whitespotted char (Salvelinus leucomaenis) was the host in the Sakasa River but not in the Chubu-Nogu River. This difference in host species suggests that there may be two Margaritifera species in Japan.

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