44 Records of the Indian Museum. [Vol, XIX, 



structure of the operculum is distinctive when it can be seen, but 

 is often obscured by deposits of algae or mineral substances on the 

 surface. These should always be cleared away before the oper- 

 culum is examined. It fits precisely into the mouth of the shell. 

 The radula differs from that of Bithynia in the following points : — 

 (i) the central tooth has a distinct quadrate projection on its disc 

 and is produced in the middle on the basal margin ; (2) the latero- 

 basal denticulations on this tooth are very few and are situated at 

 some distance from the lateral margins ; (3) none of the den- 

 ticulations of the lateral teeth are greatly enlarged ; (4) those of 

 both marginals are minute and sharp. The soft parts seem to be 

 very similar in the two genera. 



Type- SPECIES, Amnicola sistanica, Annandale and Prashad. 



Mousson's ' Bithynia ejeda from Mesopotamia belongs to this 

 subgenus, as well as the type-si^ecies and the Indian and Burmese 

 forms listed above. 



Genus Bithynia, lycach (1818). 



1919. Bithynia, Godwin-Austen, Rec. hid. Miis. XVI, p. 213. 



The shell of this genus is as a rule more elongate, more acu- 

 minate and more conical than that of Amnicola. The umbilicus is 

 usually perforate but constricted and even when it is completely 

 closed a well-defined groove can be distinguished proceeding ob- 

 liquely downwards from it but not meeting the margin of the lip. 

 The columellar fold is a sharp and prominent ridge, forming a wall 

 along the inner margin of the groove. The shell-sculpture in the 

 Indian forms is microscopic. The operculum is never very thick. 

 It is marked externally with coarse concentric ridges, encircling a 

 central or subcentral nucleus. The internal surface is nearly 

 smooth. In young specimens traces of a spiral structure can 

 sometimes be detected on the external surface. The radula differs 

 from that of Alocinma in the points noted in discussing the 

 latter genus. 



Type-species, Helix tentaculata, L/inne. 



Hydrobioides, Blanford (1869). 



1918. Hydrobioides, Annandale, Rec. lad. Miis. XIV, p. 117. 



This genus is closely related to Bithynia but may be dis- 

 tinguished by the structure of the lower part of the shell and of the 

 radula. Although the spire and the upper part of the body-whorl 

 are thin the lower part of the latter, or rather the lower and outer 

 part, is considerably thickened and has a more or less porcellaneous 

 appearance. The columellar callus is also much broader and flat- 

 ter. In several species a prominent varix on the body-whorl is a 

 conspicuous feature. The shell varies in shape so greatly that 

 two subgenera may be distinguished. The operculum is like 

 that of Bithynia, but is relatively smaller ■^ and the concentric lines 



i Mousson, Joiirn. Conchyl, XXII, p. 46 (1874). 



* It is sometimes withdrawn into the shell in highly contracted specimens. 



