I. MATERIALS FOR A GENERIC REVISION 



OF THE FRESHWATER GASTROPOD 



MOIvLUSCS OF THE INDIAN EMPIRE. 



No. 3. The Freshwater Genera of Hydrobiidae. 



By N. Annandale, D.Sc, F.A.S.B., Director, and B. Prashad, 

 D.Sc, Assistant Superintendent, Zoological Survey of India. 



The small size and insignificant appearance of the members of 

 this famih' have caused them to be generally neglected, and the 

 classification of the Indian forms in the official Fauna of British 

 India seems to be based on no principle at all. Indeed, one of 

 the genera is even placed in the I^ittorinidae, apparently through 

 inai'dvertence. The recent attempt ' of one of us to revise the 

 species assigned to Bithynia, lyeach, was not, as Mr. A. S. Kennard 

 has pointed out in a letter, sufiicientlj" drastic, for some species 

 distinct from it had been retained in the genus. For these species 

 the name Digoniostoma has been already ' proposed. We include 

 here a detailed description of this new genus. 



It will be convenient to begin our discussion of the genera 

 with a key, in which we will ignore their distribution into sub- 

 families, the diagnostic features of which are concealed in some 

 species by secondarj^ modifications in such a way that it is some- 

 times easier on first examination to recognize the genus than the 

 subfamily. The subfamilies, nevertheless, seem to be founded on 

 good anatomical as well as conchological characters. We do not 

 propose at present to discuss the estuarine and maritime genus 

 Stenothyra , Benson, which calls for a special revision, or the brack- 

 ish-water species called Bythinella or Belgrandia miliacea by Nevill ' 

 and Bithinella canningensis by Preston.* The true generic position 

 of this species will be considered best in reference to Stenothyra. 



Key to the Indian Freshwater Genera of Hydrobiidae. 



I. Shell vety small, thin, elongate, narrowly perforate or im- 

 perforate, with the columellar callus poorly developed 

 and the lip thin. Operculum thin, horny, paucispiral. 

 Central tooth of radula without basal denticulations. 

 Male organ without lateral process ... ... Tn'ciila. 



II. Shell thick, globose, with the spire directed backwards 

 and outwards, with the mouth broad and the columellar 



1 i?fic. /lid. Mas. XIX, pp. 41 — 46 (1920). 

 ^ Ind. Journ. Med. Res. (paper in the press). 

 3 Hand-List Moll. Ind. Miis. II, p. 52 {1885). 



* Ann. Mag. Nat. Hist. (7), XIX, p. 216, fig. in text (1907), and Fauna 

 Brit. Ind. Fresh'.v.-Mpll., p. 66 (1915). 



