X. INDIAN FREvSH WATER MOLLUSCS ASSIGNED TO THE GENUS BITHYNIA. By N, Annandale, D.Sc, F.A.S.B., Director, Zoological Survey of India. Ill Mr. H. B, Preston's volume on the Freshwater Molluscs in the Fauna of British India series, eighteen species are assigned to the genus Bithynia, Leach (with the two subgenera Hydrobioides, Nevill and ( ? ) Fossarulus, Neumayr) and several allied forms have since been described from Burma and Assam. All of these (22 species) I have examined, so far at any rate as the shell and operculum are concerned. I find no less than five distinct genera included among jNIr. Preston's eighteen species. It is unfortunate that in the official account of an important section of the fauna of India no attempt seems to have been made to examine these species critically. Some of them, probably the majority, must be represented in English collections. I shall not attempt at present to revise these species, but mereh^ to assign them to their proper genera and sub-genera, to point out the characteristic features of these, and to describe a new genus and a new subgenus that seem to be necessary. Of the five genera here discussed, four are closeh* related and must be included in the subfamih' Bithyniinae. The fifth, how-ever, which has recently been described by Col. Godwin -Austen, is so distinct that it may be accepted as the type of a new subfamil}', the I\l3"sorellinae. The external anatomy of the two subfamilies is identical, but there are important differences in the radula as well as the shell. The subfamilies may be briefly described as follows : — Bithyniinae. Shell never very thick, smooth to the naked eye or with spiral ridges, ovate or globose, with the peristome continuous and the colu-mellar fold ridge-like. Operculum calcareous, concentric or spiral. The central tooth of the radula usualh^ bearing a series of latero-basal denticulations on each side. The external male organ with a well-developed lateral process. Mysorei,i.inae. Shell rather thick, with strong spiral ridges, turbinate, with the peristome continuous and prominent. Operculum thick, calcareous, con-centric. The central tooth of the radula with-out latero-basal denticulations but bearing a single downwardly-directed blunt process on either side. The external male organ as in the Bithyniinae.
BioStor
