The molluscan genus Gyraulus (Gastropoda:Planorbidae) in southern Africa

Descriptions are given of the shells and some anatomical features of the freshwater planorbid snails Gyraulus costulatus costulatus (Krauss) and Gyraulus connollyi sp. nov. Topotypes of costulatus have been examined and are separable from connollyi only by conchological characters. Distribution patterns for the two species established by examination of about 15,000 snails from 1188 localities suggest that costulatus is a member of the tropical fauna, whereas connollyi is apparently endemic to the temperate climatic region of South Africa. The influence of temperature seems to be important in determining the ranges of these species. The apparent absence of connollyi from the Rhodesian highlands could be due to a barrier to northwards dispersal provided by the hot and dry Limpopo river valley.