I9I7'] J- HoRNELL : Indian species of Meretrix. 167 



coast of India from Cape Comorin to Bombay and again on the 

 west coast of the Malay Peninsula. 



The type form attains a larger size than any of the local races 

 of the living sub-species to be described later ; in the outer 

 channel of the Chilka Lake, where conditions seem particularly 

 favourable to its growth, it attains a length of 51 mm. by a depth 

 of 47 mm. while its thickness, due to its ventricose shape, is as 

 much as 33 5 mm. 



The following measurements of individuals of different ages 

 from the three chief localities where the type is found exhibit the 

 great breadth (depth) of the shell as compared with the length. 

 In variety ovum the ratio is considerably less owing to its frequent 

 greater elongation. The thickness of the complete shell shows 

 little difference in the two forms. The dimensions are : — 



Milliiiietres. Respective Ratios. 



Chilka Lake ... 5iX47X33'5o =100 to 92'i6 to 65'7o 



Pulicat ... 44X38x28 =100 to 86-36 to 63-63 



,, ... 40X33X23 =100 to 82-50 to 57-50 



Madra.s ... 39X32x21-25 =100 to 82-05 to 54-48 



,, ... 2I-5X I7-75X 12-50 =100 to 82-56 to 58-14 



,, ... 20x17-75x12 =100 to 88-75 to 6o'oo 



.\verage of ratios --=ioo to 85-73 to 59-91 



The outline of each valve is distinctly cordate (as shown in 

 figs. 30-31), but this varies considerably even in the Chilka Lake 

 individuals and some show a distinct tendency to elongation pos- 

 teriorly ; these connect with Hanley's M . ovum (pi. V, fig. 23). 



The periostracum in most of the Chilka specimens is olive 

 grey, thin and strongly adherent ; smooth and inclined to dullness, 

 it never possesses the brilliant varnish-like polish characteristic of 

 that of M. meretrix. In a few Chilka shells it appears much 

 stained with a deep rufous brown, and in one this passes into a 

 blackish brown. Shells from the bed of Pulicat Lake and Ennur 

 backwater scarcely ever retain the olive grey tinge which I believe 

 to be the natural tint of the periostracum in this species ; they are 

 almost all deeply stained with rusty brown. The single Tuticorin 

 shell obtained alive was olive grey. 



Very marked variation is noticeable in the thickness of the 

 valves, in the strength of the hinge plate, and in the size of the 

 cardinal and lateral teeth in specimens from the east coast of 

 India ; the series shown on plate VI, figs. 30-33, illustrate the range 

 in form better than any verbal description. In no case, however, is 

 the shell and hinge so massive as in the sub-fossil variety described 

 below as var. satparaensis. 



On the west coast of India from Travancore as far north at 

 least as Bombay, a variety of M. casta is found in every estuary 

 and backwater in such immense numbers that it has acquired a 

 position of considerable economic importance among the fishing 

 community, of whom hundreds engage in its collection during the 

 dry season when the level of water is low in the channels. The 



