Abstract
A freshwater snail, genus Semisulcospira was studied in the three islands inside Lake Biwa, Japan. A large and pronouncedly nodulated form of S. niponica was found off Takeshima and Shiraishijima Islands as well as off Chikubujima Island from which this form was described. Among the three islands, the snails become larger and nodes on the whorls become more intense with the decreasing area of the islands.
A new species, S. morii, is described from Chikubujima and Takeshima Islands. This species coexists with S. niponica, and is abundant on a rather deeper bottom according to the monthly survey of vertical distribution. Three unknown types of Semisulcospira were also found off Takeshima and Shiraishijima Islands. All of the types live on the rocky bottom there, but in adult and embryonic shell characteristics they resemble the decipiens group which live on the sandy or muddy bottom of the coastal zone of Lake Biwa. From the above survey, several ecological and evolutionary problems are presented on Semisulcospira in these islands.
