1 92 1. J Maniptir Molluscs. 625 



dium clarkeanum , burrowing species, were found only at the ex- 

 treme edge of tfie northern part of the lake where the vegetation 

 is less congested, while the third bivalve {Sphacritim indicum). 

 which swarms freely among the branches of water-weeds, was most 

 abundant in the deeper parts. 



Linwaea ovalior probably occurs in the Loktak Lake when it 

 is full as we found it in small pools that would be included at that 

 season, but it is even more of an exclusively paludine species than 

 those discussed as inhabitants of the lake. Indeed, it seems to be 

 almost amphibious in habits and thus from an ecological point of 

 view may almost be classed with Succinea elegans, a species found 

 in abundance at the edge of the northern part of the lake. 



Only a few species were found in running water, but here it 

 is necessary to recognize a fundamental difference between the 

 rapid-running streams of the hills, with their clear water and stony 

 bed, and the sluggish, turbid rivers of the valley. In hill-streams 

 the only Gastropods commonlv observed were Paludomits piistiilosa 

 and the narrowest phase of Limnaea andersoniana. Bivalves were 

 rather move common and included the following species, Corbicula 

 occidens, Indonaia bonneaudi , I. theobaldi and /. lima^ all thick- 

 shelled forms, as is also P. pustulosa. At least two other species 

 make their way into mudd}', comparatively still pools in such 

 streams, viz. Melanoides tuberculatus and Acrostoma variabilis. 



In the larger rivers of the valley the muddy bottom is fa- 

 vourable to these two Melaniidae and also to the thin-shelled Uni- 

 onidae of the genus Layndlidens^ while in small, sluggish stream- 

 lets and water-courses Ancyliis viola, Limnaea acuminata, Corbicula 

 occidens and Pisidium clarkeanum are sometimes not uncommon. 

 it was in such a streamlet also that we found Cam-ptoccras liwatum. 



Generally speaking, the species of Palitdomns, Acrostoma and 

 Indonaia are inhabitants of running water. Paludoinus is found 

 as a rule in mountain streams or at any rate in running water near 

 the base of hills and on a stony bottom, while Acrostoma and In- 

 donaia need mud and therefore less rapid water. As is suggested 

 in Dr. Baiui Prashad's part of this paper, the genus Lamellidens 

 can probably be divided into two sections from an ecological point 

 of view, one, which produces very large numbers of embryos and 

 as a rule frequents running water, the other, with a smaller number 

 of embryos, that affects ponds and swamps. These observations, 

 to which there are of course exceptions, are on the whole substan- 

 tiated in Manipur, but in applying them it must be remembered 

 that conditions in a very sluggish, weed-choked stream often 

 approximate closely to those in a swamp and attract paludine 

 forms. 



Variation and Plasticity. 



It is particularly interesting to contrast the Manipur Valley 

 with that of the Inle Lake in reference to the variability and plas- 

 ticity of the aquatic molluscs. As I have pointed out in the Intro- 

 duction to this paper, the two valleys have certain physical fea- 



