X. A REVISION OF THE INDIAN SPECIES 

 OF M ER ETRIX . 



By James Horneli., Marine Biologist to the Government 

 of Madras. 



(Plates IV-VII.) 



Preliminary. 



The genus Meretrix was split off from Cytherea to accommo- 

 date a number of strongly marked species distinguished character- 

 istically by the possession of an elongate and finely striate (or 

 granulate) posterior lateral tooth — the nympha of older writers — 

 in each valve. They frequent estuaries and none of those found 

 on the coasts of Continental India live beyond the influence of land 

 drainage ; they have about an equal tolerance with the backwater 

 oyster [Ostrea virginiana) for an occasional declension in the salin- 

 ity of the water of their habitat. They can also endure consider- 

 able increase in salinity. This tolerance is of limited time-dura- 

 tion in both cases. Within my knowledge both of the common 

 species can survive, at least for some days, a lowering of salinity 

 to I'oio S.G. and again can endure a concentration during the dry 

 season that may reach as high as i'030 S.G. How long such ex- 

 tremes can be borne we do not know, but it appears certain that 

 vitality is lowered in either case and if the abnormal conditions be 

 not modified within a certain period, widespread death supervenes 

 in the beds. The -optimum range of salinity favoured ranges from 

 1-025 to 1-027 S.G. 



My attention was directed to this genus during an investiga- 

 tion of Indian mollusca of economic value ; the examination of large 

 numbers of these shells showed that great variation exists and 

 when attempting to identify the various forms it became apparent 

 that this liability to vary widely, both in form and colouration, has 

 resulted in great taxonomic confusion and the undue multiplica- 

 tion of species. Examination of the Indian Museum collections, 

 kindly placed at my disposal by Dr. Annandale, emphasized the 

 need for a revision of Indian species and the consideration of what 

 value and limits should be placed upon the many variations which 

 obviously exist. 



The material which has furnished the data for this revision 

 was derived in the main from the shell collections in the Indian 

 Museum, especially that obtained during the zoological survey of 

 the Chilka Lake, and from extensive collections which I have 



