i')2i.] Manipur Molluscs. 539 



the fauna may be regarded on the whole as normal in composition. 

 Moreover, with the exception of the Viviparidae, the species are 

 but little modified. The majorit^r of them are, indeed, of wide or 

 fairly wide geographical distribution, and, considering the isolation 

 of the valley, perhaps less remarkable than might be expected. 



The Viviparidae, however, are not only to a large extent 

 etidemic in the valley, to which two of the four species are apparent- 

 ly confined, but also peculiar in anatomical structure as well as in 

 shell-sculpture. Two of the four are ornamented with smooth 

 spiral ridges on the shell, while I have been obliged to institute 

 for a third a new genus, founded mainly on the structure of the 

 operculum and mantle. This species {Lecythoconcha lecythis) is, 

 however, by no means endemic in Manipur, having a wide range in 

 south-eastern Asia and belonging to a group essentially Chinese in 

 distribution. 



Family HYDROBIIDAE. 



Three species of this family have been found in Manipur, all in 

 the valley. Two belong to a genus recently described as new under 

 the name Digoniostoiua, but widely- distributed in India proper 

 and Assam; while the third represents the subgenus Alocinma, 

 recentlj- set up as a subgenus of Amnicola b^' Dr. Baini Prashad and 

 myself with a Persian species as type. This genus has a wide 

 range, which extends at any rate from Mesopotamia to Upper 

 Bunna. 



The three genera Bithynia, Leach, Digoniostoma , Annandale, 

 and Hydrobioides, Nevill, and the subgenus Alocinma, Annandale 

 and Prashad, are so closely allied and so liable to be confused that 

 it will be well to give a key to them here. The anatomy of all 

 is very similar and they are distinguished mainly by the structure 

 of the aperture of the shell and of the operculum. 



A. Periostome continuous ; outer lip neither thickened 



nor attenuate; innbilicus closed or rimate, without 

 an oblique channel running forward on the lower 

 surface of the shell ; operculum luith a distinct 

 but paucispiral figure situated near tire middle of 

 the lotiiey part and visible on both surfaces ... Alocinma. 



B. Operculum concentric, with no spiral figure on the 



lower part or visible on both surfaces. 

 i. A well-defined oblique channel running for- 

 wards from the umbilicus on the lower 

 surface of the shell. 



a. Outer lip thin, not produced or 



angulate at its inner extremity ... Bithynia. 



b. Outer lip slightly thickened, pro- 



duced and angulate or subangulate 

 at its inner extremity ... Digoniostoma. 



ii. No well-defined channel running forwards 

 from the umbilicus. Outer lip distinct- 

 ly thickened, but not produced at its 

 inner extremity ; a supplementary varix 

 often present outside the thickened lip. Hs'drobioides. 



