1921.] Manifur Molluscs. 623 



genus, V. crassispiralis to the Viviparae bengalenses and V. mic- 

 ron to the Viviparae dissimiles. Each species, however, is quite 

 distinct from any other, as are also D. textwn and S. elegantior, 

 the resemblance between the shell of the latter and that of the 

 Indo-Burmese S. scmiscrica being superficial. The Pcdudomus, 

 on the other hand, is closely allied to P. conica, a remarkablj' 

 plastic Assamese species with many local races, amongst which 

 the Manipur form might perhaps be included. 



Considered as a whole the Gastropod molluscs of the ponds, 

 swamps and streams of Manipur are thus remarkable from a 

 geographical point of view in only one feature, in the small 

 evidence they afford of a close connection with those of Burma 

 such as might have been postulated from the fact that the river- 

 system of the Manipur Valley, in which the great majority of 

 them live, is directly connected with the largest tributary of 

 the Irrawadi and completely isolated from all other systems. 



We may now consider the geographical distribution of the 

 bivalve molluscs of Manipur. Among these si.K genera are repre- 

 sented, Indonaia, Lamellidens ^ Trapczoideiis, Corbiciila, Sphaeriuin 

 and Pisidiiim. The first three genera belong to the Unionidae, 

 the last three to the Cyrenidae. As the two families have 

 different means of dispersal and also different limitations in 

 their dispersal, we may consider them separately. The parasitic 

 period in the life of the Unionidae and the fact that the different 

 species are attached to different species of fish in this period give 

 the members of the familj' a jieculiar means of progression from 

 one part of a river-system to another and at the same time corre- 

 late their geographical distribution with that of their hosts. 

 We might expect, therefore, that the Unionidae of the Manipur 

 Valley would be more exclusively Burmese than either the Gastro- 

 pods or the Cyrenidae. Mr. Sunder Lai Hora, who has worked 

 out the large collection of fish he made in Manipur, tells me that 

 he finds among them a large proportion of Burmese species and 

 that he obtained evidence, direct and indirect, that certain species 

 migrate up the luiphal River at certain seasons. That such fish 

 should bring with them from Burma the glochidia of Burmese 

 Unionidae would be what might be expected. But the evidence 

 for this is not very strong. The genus Trapezoideus is certainly 

 m the main a Burmese and Indo-chinese genus and the only 

 species found in Manipur [T. misellns) is a Burmese and Indo- 

 chinese species, but the occurrence of another, hitherto undes- 

 cribed species {T. dhamishori , Prashad) north of the Naga Hills con- 

 siderably discounts the value of this piece of evidence, though 

 it does not run counter to it. Indonaia, although it has its 

 headquarters in the north-eastern part of the Indian Empire, is 

 by no means exclusively Burmese. Three of the four species 

 found in Manipur have also been found in Assam if not in India 

 proper, and only two of these in Burma, while the fourth is known 

 only from the Manipur \' alley. 



Even from the Unionidae, therefore, evidence for any but a 



