A new speices of the family Turridae, Oenopota sagamiana is described. This species aggregated on the metachromatic area of the sea bed near and within the Calyptogena-community at cold seepage site in Sagami Bay.

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Cuna iriomotensis n. sp. is described. This is a range extention of the genus from Australia up north to Okinawa, Japan.

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Twenty-seven species of the Subfamily Oenopotinae of the Family Turridae were identified in the materials that had been previously reported by Ito (1967, 1985, 1989) and Ito et al. (1986) from Japan Sea. Among them 21 species among which three are new to science are described and illustrated. The most of them are widely distributed subarctic species, but ranges of a few species seem to be limited in Japan Sea coast of Honshu.

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A new species (Pyramidelloides barbadensis n. sp.) is described from deeper waters around Barbados. Another new species belonging to the genus Coenaculum, a genus up to now only known from waters around Australia is described from Belize.

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Taxodont bivalve, Ryukyuleda japonica, n. gen. et n. sp. having resemblance with the species of Eocene Hilgardia in the eastern U.S.A. was described from the Pliocene Shinzato Formation in Okinawa, Southwest Japan. Autochthonous occurrence of the species is also discussed by micro-borings and gastropod penetration preserved on its external shell surface.

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Seasonal changes in the moving activity of Clithon retropictus were studied from the distance moved by this snail during one month as an indicator. In the upstream portion of blackish-water zone in the Naka river of Izu Peninsula, a total of 241 marked individuals were released from May 1989 to November 1989. Recapture was undertaken once a month from July 1989 to September 1990 in order to survey the distance moved by marked individuals. The distance moved from release site was recorded by counting the number of square-shaped pools which are formed between large cross-shaped concrete blocks arranged parallel to the side wall of this river. Out of recapture records, only data of the individuals recaptured one month after release were extracted to study monthly changes in mean and maximum distances. Both mean and maximum distances were large from April to August with slight decrease in June, but decreased from September to March showing the smallest in December and January. Sexual difference of monthly changes in mean and maximum distances was analysed using 33 males and 35 females recaptured. Monthly mean distances moved by males were large in February and May to August, especially larger than those by females in February, May and June. This active movement suggests that reproductive activity of males searching for females to transfer spermatophores in copulation has already begun actively before the active egg-laying period from April to June. It also suggests that active movements after the active egg-laying period are related to the growth after the end of reproductive activity. Monthly changes in the maximum distances moved by males showed a similar pattern as in the mean. On the other hand, the mean distances moved by females showed gradual increase from February to August, showing slight decrease at two peaks of egg-laying in April and June. This slight decrease in the mean distance during active egg-laying period suggests that this period is devoted largely to egg-laying behavior. The mean and maximum distances moved by females in July and August after the peak of egg-laying were larger than by males, and this suggests that females show large growth in this period. Studies on monthly changes in the relation between the distance and the shell size during active moving season from February to November showed that the distance does not increase with the size of individuals but the maximum distance was gained by middle-sized individuals especially during egg-laying season from February to August. Therefore, the egg-laying season was divided into the first period (February to June) from the beginning until the second peak of egg-laying when large energy consumption for egg-laying is expected and the latter period (July to August) when large growth after the end of egg-laying activity is expected. The relation of the distance to the shell size was compared between males and females in these two periods. The maximum distance moved by males was observed in the middle-sized individuals having the shell length of 14.35 mm during the first egglaying period, but during the latter period the maximum one was observed in larger individuals having the shell length of 16.40 mm. The distances moved by small-sized and large-sized individuals were larger in the latter period than in the first one. Increase of the distances moved by small-sized and large-sized individuals in the latter period is considered as a result of lowered reproductive activity. The transition of the shell size showing the maximum distance from the small-size to the large-size suggests a heavy burden on reproductive activity in the small-sized and large-sized individuals during the first egg-laying period. On the other hand, nearly similar tendency was observed in females as in males. The distance moved by large-sized females were larger than by males in the latter period. This suggests that females grow larger in shell size than males.

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Growth and number of newborns in Semisulcospira reiniana reared during 48-51 months under the three different temperature conditions in the laboratory were studied. The newborns, which were produced in July and August 1986, grew up to 8-9mm in mean shell diameter by the age of 1, 11-12 mm by the age of 2, 12-13 mm by the age of 3, 13-14mm by the age of 4. The maximum size of S. reiniana reared at 21±2℃ is 16.4 mm in shell diameter, 45.1 mm in shell height, 7.4 g in wet weight with shell. When the size of mature snails grown up at each aquarium reached over 9 mm in mean shell diameter, 20-22 mm in mean shell height, 0.9-1.0 g in wet weight with shell at the 12-14th months since their births, the production of newborns was begun. Birth of newborns occurred at a water temperature between 12℃ and 28℃. Large number of newborns were produced at a water temperature between 20℃ and 28℃. Number of newborns during a month per female is estimated 10-38 individuals at a water temperature between 20℃ and 28℃. Large number of newborns during a year were produced by mature female larger than 10 mm in mean shell diameter at 2-3 ages. Optimum temperature for growth is lower than for production of newborns. Under the high water temperature condition, maximum size of snails was small and survival rate was low, but production of newborns was increased.

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The Neogastropoda had been recognized as dioecious having the penis in the male but no penis in the female. Since early 1970s, however, many species have been found showing the phenomenon of "imposex" in which male characters such as the penis and vas deferens are superimposed on the female. Nakano and Nagoshi (1980) found the same phenomenon in Thais clavigera collected in Shima peninsula from 1976 to 1978. Therefore, in order to infer the cause of this phenomenon, we investigated the local variation in the frequency and intensity of imposex in the females of T. clavigera collected at 9 localities of Japan, i.e., Asamushi, Oga, Yokosuka, Shimoda, Toba, Shirahama, Hiroshima, Amakusa and Kagoshima, from 1978 to 1980 and at Hong Kong in 1982. As a result, the imposex was found in all localities of Japan with various frequency and intensity but not in Hong Kong. The imposex has been confirmed by many researches since early 1980s to be induced by tributyltin (TBT) contamination from antifouling paints used for ships and boats. Therefore, the results of the present study suggest that coastal waters in many localities of Japan had already been contaminated with tributyltin from antifouling paints about 10 years ago.

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There are three subfamilies among the family Fasciolariidae, namely, Fasciolariinae, Fusininae and Persterniinae. These three subfamilies are recognizable by conchological characters. In this sturdy, the types and structures of the osphradium, radula, habit and habitat of 13 species of the Fasciolariidae (1 species of Fasciolariinae, 6 species of Fusininae, and 6 species of Peristerniinae) were investigated. Fasciolariinae has a large shell (>10 cm), Type IV osphradium with 170-220 leaflets, and 17-22 cusps of lateral teeth of the radula. Fusininae have large to middle sized shell (>8 cm), developed Type V osphradium, but the numbers of osphrdial laeflets and those of cups are not so large. These two subfamilies seem to be efficient hunters. Fasciolariinae live on the sandy or rocky bottom at about 20 m deep, and prey on large bivalves and gastropods. On the contrary, Fusininae live in sandy or on rocky bottom at about 20-100 m deep and eat perhaps polychaetes. Peristerninae has a hard shell of the middle to small in size (<10 cm), Type IV osphradium with 70-130 leaflets, and lateral teeth with 5-10 cusps. They live on coral or rocky bottom in 0-20 m deep and capture invertebrates. Granulifusus resembles general Fusininae in shell form, but it is smaller (about 4 cm) in size, and has Type IV osphradium with 60 leaflets, and lateral teeth with 5-6 cusps. It lives sandy bottom in 100 m in depth. Opercula of three subfamilies are corneous and downward situated nucleus, but only Granulifusus has the small and oval operculum with the nucleus lying on lateral side. So, it would better create a new subfamily to this genus.

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Mikawa Bay is located in the Pacific coast of central Japan and it is one of the representatives of Japanese shallow embayment. It consists of two branched bays ; Chita Bay in the north and Atsumi Bay in the south. On June 24 and 25, 1962, molluscan remains and living benthos were collected at 31 sampling points in Mikawa Bay by Itoigawa et al. They used Smith-MacIntyre type sampler which is capable of scooping sediments about an area of 32×35 cm. This study is the second report on molluscan thanatocoenoses of Mikawa Bay using the material. In the previous report (Nobuhara et al., 1991), we studied on thanatocoenoses of bivalves and classified them into six types indicating marine environments of each part in Mikawa Bay. In this report, we study shell remains of gastropods and scaphopods. We identified 113 species in 93 genera of gastropods and 4 species in 3 genera of scaphopods. In particular, we examine quantitative distributions of gastropod shell remains. We adopt the following two ways of counting the shell remains of gastropods ; counting an apex part as one individual and counting a specimen having more than half of shell. As a result, we recognize three types of distributional pattern common to many species as follows ; seaweed bed type, embayment type, and outer bay type. The species group of seaweed bed type mainly deposits its shell remains in the eastern central to inner part of Atsumi Bay, where the depth is less than 10m. This species group is characterized by phytal species such as Diffalaba picta vitrea. The species of embayment type deposit much shell remains in the eastern central part of Atsumi Bay and the remains radically decrease in Chita Bay, the inner part (13A, 14A) and the western central part (the depression of the central bay area) of Atsumi Bay, where oxygen content becomes seasonally very low. The species of outer bay type show the distribution restricted to the outer side and the mouth of Mikawa Bay. The distributional pattern of the gastropod shell remains generally corresponds to embayment degree indicated by the thanatocoenoses of bivalves. But there is no gastropod species which adopt to water mass of low oxygen content, such as Theora fragilis in bivalves. Moreover, we cannot recognize oceanic gastropod species which extend their distribution to the inner side of the bay, such as Microcirce dilecta in bivalves. However, changes of quantity and species composition of gastropods in thanatocoenoses correspond to seaweed bed, water mass of low oxygen content and oceanic water. These changes serve to make a cross-check of environmental analyses with index species of bivalves.

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The chromosomes of Acila (Truncacila) insignis in Nuculidae and Yoldia (Cnesterium) notabilis in Nuculanidae were studied. In A. (T.) insignis, chromosome numbers of 24(2n) and 12(n) were observed, and in Y. (C.) notabilis, 38(2n) and 19(n) were counted. Karyotypes were as follows : A. (T.) insignis (2n=24) : 18 m+2 sm-m+4 sm, NF=48. Y. (C.) notabilis (2n=38) : 18 m+10 sm+2 st-sm+8 st, NF=76.

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