INTRODUCTION. 



In collecting- the materials t'oi- the following pages, the Author 

 has heeu chietly coucerned in assembling, as far as possible, the 

 original descriptions of all tlie freshwater species of both 

 Gastropods and Pelecypods inhabiting the regions in quesliou. 

 Many of these, however, are of so short or fragmentary a nature 

 that it has been deemed advisable either to supphmt them 

 altogether or to supplement them considerably by various notes 

 of his own or of other writers oil the subject. Moreover, even 

 original descriptions have only been retained when printed in 

 English or Latin, those in other languages having been either 

 translated or rewritten. 



An effort has been made to reduce all original measurements, 

 as far as possible, to millimeters, and thus to ehminate a host of 

 different systems ranging from English inches to German lines. 



Wherever possible, illustrations of hitherto untigured species 

 have been given, though in a few cases, owing to the lack of 

 authentic specimens, this has not been found practicable 



In some cases considerable difficulties have presented themst'lves 

 with regard to the inclusion or rejection of certain genera which 

 include both fresh and brackish-water forms, but generally 

 speaking a genus has been included where members are liable, 

 when occasion arises, to adapt themselves readily to new con- 

 ditions, such, for instance, as in the case of the ponds and pools 

 of the Gangetic Delta, which, though normally brackish, may 

 become practically fresh through excess of rainfall after a dry 

 season, or througli the gradual silting up of the saltwater chan- 

 nels feeding them ; in such instances the Mollusca which inhabit 

 them will generally thrive equally well under the new conditions 

 and become, in effect, freshwater species. 



At one time it was intended to add to the present volume the 

 terrestrial operculate genera, and a certain portion of the work 

 was actually done when the Author was informed that this group 

 had already been entrusted to other hands ; under the circum- 

 stances the work was immediately stopped and, presumably, these 

 families will form the subject of another volume by a different 

 author. 



It is nnich to be regretted that the present volume can, in most 

 instances, only deal with the shells of the species quoted, this 

 being largelv due to the little anatomical work which has been so 

 far done on the freshwater genera inhabiting the Indian region 



