Various macroscopic and microscopic characters of propeamussiids from Japan were examined, and their taxonomic and evolutionary significances, though preliminarily, were considered. Five species of Propeamussium, two species of Polynemamussium and two species of Cyclopecten from Japanese waters surely share the diagnostic characters of the Propeamussiidae Abbott, 1954 [emend. Waller, 1978], i.e. flexible ventral margin of the right valve (owing to the single layer of prismatic calcite), discrepant surface sculpture and shell microstructure between the two valves, absence of true ctenolium even in early growth stages, and imbedded pallial myostracum in the inner layer of crossed lamellar aragonite. In some species it was clearly observed that each prism of the outer layer of the right valve is internally foliated, and that fibrous prismatic structure gradationally change into foliated structure. The morphological resemblance between Propeamussium and Amusium is only superficial because their internal ribs are genetically unrelated. Diversified species of the Propeamussiidae in the western Pacific may be important for understanding the classification and evolutionary process of the suborder Pectinina, because they seem to retain various primitive characters as "living fossils".

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The species Lamellidoris (Lamellidoridella) mitsuii Baba, 1938 from Japan was re-identified by Millen, 1985 as Diaphorodoris mitsuii. This paper describes the anatomy of that species fully.

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A population of small-sized individuals of Nautilus from the Sulu Sea is thought to represent an intraspecific form of N. pompilius. This new subspecies differs from the typical one in having smaller shell with coarse color stripes with more purplish hue.

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Genital system was studied for intraspecific varaition in a camaenid land snail, Satsuma tanegashimae (Pilsbry, 1901), which is distributed in the northern parts of the Ryukyu Islands. In the genus Satsuma the genital system is in structure rather stable. Some characters such as the shape of appendix of epiphallus and flagellum, however, vary among populations of the same species or even between individuals of the same population, so that they cannot always be used in discriminating between species. Interpopulational variation of genital system is not great in S. tanegashimae, which is constantly distinguished by a very long flagellum from its congeners. The Uji population has a somewhat peculiar genital system with a long appendix of epiphallus and small bursa copulatrix sac. The Kusagaki population has small genitalia for its large shell. Discriminant analysis was made to estimate the distances (Maharanobis D-square : MD) between populations based on genital system characters. Cluster analysis using MD-values suggested that three groups (Uji, Kusagaki-Mishima-Tokara, and Tane-Yaku) exist within this species, and that the Uji population is quite peculiar. The Kusagaki population may have been derived from a stock on the Tokara Islands.

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The field and indoor experiments were carried out to elucidate the habitat preference, diurnal activity pattern and predators for the artificially bred juvenile abalone Haliotis discus discus Reeve at Kominato, Chiba Prefecture. For the field study, 1688 juveniles (mean shell length : 15.29 mm) were released to an artificial reef which was constructed with 50 L-shaped concrete blocks covering an area of 20m^2, and the number of individuals remained in the reef were monitored for about five months. After release, the number decreased rapidly ; 10 days later, only 339 (20.1% to the total number released), and about 3 months later only 12 individuals (0.7%) were found in the reef. After that, without much change in number, the reef kept quite a low density of the released juvenile population. Such rapid decrease and low density were examined in relation to the biological and physical conditions in and around the released site. The main cause appeared to be attributable to the lack of adequate micro-habitats for the juveniles, as the reef was constructed in the area under the strong influence of sand which was well indicated by the vegetation of Sargassum ringgoldianum growing predominantly in this sandy condition. For the indoor observation on the diurnal activity pattern of juvenile abalones, 50 individuals were placed in the experimental tank with a shelter, and then records were taken, at regular intervals, for the number of individuals remained outside the shelter in two consecutive days. Most of the individuals stayed within the shelter with little sign of their activity during the daytime, but as soon as the darkness comes they started to move out of the shelter. Three to four hours later the number of juveniles outside the shelter reached a maximum in the day, and thereafter decreased towards the dawn. Thus, they clearly showed the nocturnal habit in activity. The predation upon juvenile abalones were examined for eight species of animals which commonly occurred in the place where the field experiment was undertaken. Of these, a starfish Coscinasterias acutispina, and two crabs Thalamita prymna and Etisus electra, were observed to prey on juvenile abalones. The damage to the shells differed among the predators examined, and the killed juveniles found at the released site also exhibited various degrees of shell damage. It was inferred that the predation would partly be responsible for an abrupt decline in the number of the released juveniles. Based on the results obtained not only in this study but also previous works by various authors, the ecological features of H. discus discus at a juvenile stage were reviewed.

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There are three subfamilies among the Family Naticidae, namely, Naticininae, Polinicinae and Sininae in Japan. These three are easily recognizable by differences of shell form and operculum. The structure and number of leaflets of the osphradium of 7 naticid species were examined. The number of osphradium leaflets of Tectonatica janthostomoides (Naticinae), Glossaulax didyma (Polinicinae) and Sinum javanicum (Sininae), are 110-96, 80-76 and 58-57, respectively. The Naticinae has a simple structure, Polinicinae and Sininae have complicated leaflets. The shell of Naticinae is spherical while that of Polinicinae is hemispherical and that of Sininae is depressed. The Naticinae has a flat propodium, while Polinicinae and Sininae have developed propodia which can envelope the shells forming a shoe-like appearance that is most efficient for burying. The Sininae cannot withdraw the soft part into the shell. The members of Naticinae live near the surface of a sandy bottom, while Polinicinae bury decp in a sandy bottom. Within Naticidae, the number or complexity of the osphradium leaflets are correlated to habitat. The increased complexity might enable burying forms to utilized limited water in the interstices of sand more efficiently. On the contrary, the increased number of osphradium leaflets seem to be characteristic of shallow burying forms that can utilize abundant environmental water.

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The comparison of the amino acid compositions of the water-soluble and water-insoluble matrices in molluscan shells was made between the nacreous and prismatic layers of seven different species. The amino acid compositions of the Ca-binding glycoproteins in the water-soluble matrix of the marine Pelecypoda and Gastropoda were similar to each other, regardless of the ultrastructure of shell and of the taxonomic position, while those of the fresh-water Pelecypoda were different from those of the marine species. On the other hand, the amino acid compositions of the unfractionated water-insoluble matrix in the nacreous layer were separated from those in the prismatic layer and they also differed slightly according to the taxonomic position, while no distinct difference could be noticed between the marine and fresh-water species. The compositions of the Ca-binding glycoproteins and unfractionated water-insoluble matrix in both shell layers of Cephalopoda was unique as compared to those of the other classes. These data imply that the specific component in the water-insoluble matrix may play an important role in determining the ultrastructure of molluscan shell.

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