[in Japanese]

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The present paper describes a new species of Glycymeris (s.s.) from Ryukyu Islands, southwest Japan. The species has been known by Japanese malacologists under the name of Glycymeris "spurca (Reeve, 1843)". The new species, G. shutoi, however, differs from G. spurca by lacking finely striated primary ribs. Superficially, G. shutoi also resembles Glycymeris reevei (Mayer, 1868), G. fringilla (Angas, 1872), and G. amamiensis Kuroda, 1930 from various localities in the Western Pacific waters. Except for G. fringilla, G. shutoi, G. reevei and G. amamiensis are sympatric in waters around Ryukyu Islands and can be discriminated from each other by the differences of some characters, such as shell ornamentation, shape of ligamental area, and numbers of ventral crenulations. According to a biometrical analysis based on a thanatocoenosis of glycymeridid shells from Miyako Island, G. shutoi and G. reevei seems to represent two Mendelian populations respectively, because the frequency distribution concerning several quantitative characters appears to be a normal distribution in each species. This new species also differs from G. fringilla from Queensland by numbers of ligamental grooves and ventral crenulations.

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[in Japanese]

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The age structure and growth of Monodonta labio (Linnaeus), a common prosobranch found in many intertidal rocky shores of Japan, were studied on a population of Imaura coast of Toba City on the Shima Peninsula. Samples were collected from an intertidal cobble zone once a month from December 1977 to January 1979. The frequency distribution of shell height in each collection was analyzed by the method using probability paper to consider the age structure and growth in the sample population. In the size frequency distributions of samples from December to September, three modes are distinguished, but in those of October and November four modes are discerned. That is, the population of this species consists of three age groups of 0, 1 and 2 year-classes during the months from December to September, but in October and November it consists of four age groups of 0, 1, 2 and 3 year-classes. In all age groups, the growth continues from autumn to early summer, but during the summer the growth is retarded probably due to high temperature and spawning activity. The young snails showing the mean shell height of 1.7 mm start a benthic life from October, and they reach to the mean shell heights of 7.4 mm after one year, 14.1 mm after two years and 19.7 mm in June of the third year. Most individuals seem to die after spawning in the summer of the third year, and the life span of this species is estimated at about three years.

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Aplysia juliana at different growth stages from the hatched veliger to the adult was investigated to clarify ontogenetic allometries of the maximum shell length to the expanded body length, of the maximum shell length to the maximum aperture breadth, of the maximum shell length to the maximum aperture length, and of the maximum aperture length to the maximum aperture breadth. The relation between the shell length and the body length was expressed by two straight lines which intersected at a fairly grown juvenile stage causing the loss of larval operculum. This indicates that, in the planktonic veligers and the operculum-bearing early juveniles which have finished metamorphosis, the shell length increases in proportion to the body length. Whereas, in the juveniles after the loss of operculum and the adults, the body grows at a faster rate than the shell. On the other hand, both of the relations between the aperture length and the shell length and between the aperture breadth and the aperture length were expressed by three straight lines which intersected at the metamorphic stage and at a juvenile stage slightly after the loss of operculum. However, the relation between the aperture breadth and the shell length was expressed by a straight line throughout the life cycle of the animal.

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Diagnostic characters of four species studied are summarized as follows : 1. Zaptyx kikaiensis (Pilsbry, 1905) from Kikaijima and Tokunoshima. (1) There are two short sutural plicae. (2) Upper palatal plica extremely short, barely connected with the lunella, which is almost straight. (3) Clausilium rather long, parallel-sided, rounded at the apex, deeply excised at the columellar side of pedicle. 2. Heterozaptyx oxypomatica (Pilsbry, 1908) from Amami-Oshima. (1) Shell without sutural plicae. (2) Upper palatal plica somewhat longer, the lunella jointing it in the middle. (3) Apex of clausilium somewhat acuminate and projecting, as though a drop hanging. (4) Receptaculum seminis is about half as long as the diverticulum. 3. Metazaptyx hachijoensis (Pilsbry, 1901) from Izu Islands (Niijima, Miyakejima and Hachijojima). (1) Principal plica short and two short sutural plicae lying above it. (2) Upper palatal plica exceeding short and jointing the lunella. (3) Lunella rather long and straight. (4) Clausilium wide and rounded at the apex. 4. Metazaptyx daemonorum (Pilsbry, 1902) from Tokunoshima and Amami-Oshima. (1) Upper palatal plica is very short and connected with the very oblique and curved lunella. (2) Clausilium is short and deeply excised near of pedicle. (3) Diverticulum of genitalia is far longer than receptaculum seminis.

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[in Japanese]

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