The present investigation reveals monthly collection of molluscs in Asan river system from Sahaspur to Herbertpur between April 1994 to March 1996. As many as 18 species of molluscs comprising 13 species of gastropods and 5 that of bivalves were collected. The gastropods were grouped under 5 different families (Viviparidae, Lymnaeidae, Thiaridae, Planorbidae and Pilidae). While the bivalves belonged to 2 different families (Unionidae and Corbiculidae). Among the gastropods, the Thiarids were common and their number constituted about 42% of the total collection of the snails. Further, the following species of snails, viz., Lymnaea auricularia, L. acuminata and Thiara (Melanoides) tuberculata have been found to carry larvae of trematode parasites.
Pokhriyal et al. (Mon,) studied this question.