156 Records of the Indian Museum. [Vol. XIII, 



This species is easily distinguished from M . casta and its varie- 

 ties by the greater superficial dimensions of its valves when fully 

 grown, by its less ventricose and more compressed form, particu- 

 larly marked in young specimens, and by the delicacy and compara- 

 tive weakness of the hinge region. Apart from these differences, 

 an unfailing distinction is found in the form of the anterior cardi- 

 nal tooth of the left valve. In M . meretnx this tooth has the 

 summit distinctly notched, recalling in some degree the bifid form 

 characteristic of Tapes ; in M. casta it is invariably entire. Apart 

 from colouration, this species is remarkably stable in form and 

 general proportions; how it could be confounded with M. casta 

 and M. casta satparaensis by Preston is difficult to understand. 

 That it was, is shown by reference to the specimens identified by 

 this authority now in the Calcutta Museum and by reference to 

 the article cited above, where Preston gives the size of one in- 

 dividual as 73 > 67 mm., a size never attained by M. casta, 

 whereas these are the normal adult dimensions in the case of M. 

 meretrix. 



Outline and size of the valves. The valves are sub-trigonal and 

 vary from a broadly cordate to a sub-orbicular form, the anterior 

 angle being always well-rounded while the posterior is distinctly 

 but bluntly angular. It is exceedingly difficult to describe in 

 words the form of the shell in the different species and varieties of 

 this genus sufficiently clearly to convey an adequate comprehen- 

 sion of the differences between them. Hence reference must be 

 made to the figures which accompany this note. The dimensions 

 of the shell attained by M . meretrix when fully grown are remark- 

 ably constant; the largest noted (Tuticorin| is 77 X 65 X42I mm. 

 thick ; others from the same locality measure 77 X 64 X 41 mm. thick 

 and 74X65x40 mm,, while three from the outer channel of the 

 Chilka Lake measured respectively 73 X 67 mm. (M. 9582/2),68 X 6o|- 

 X 42 mm. (M. 9762/2), and 65 X 58 X 40^ mm. (M. 9763/2). The aver- 

 age of size at full maturity for the valves appears to range from 

 58 to 60 ram. in depth by 65 to 70 mm. in length. Shells of these 

 dimensions are those most commonly found when collecting ; the 

 animal at this size has reached its limit of growth and this again 

 coincides closely with the age-limit of its life. Having spawned 

 after attaining this size, its vitality appears to ebb and I have 

 noted the death of large numbers at Tuticorin after the Septem- 

 ber spawning ; during October the sand flats in the lagoon are 

 thickly dotted with dead and gaping shells of full-grown size. It 

 is noteworthy that small and medium-sized dead shells are conspi- 

 cuous by their absence, although living individuals are present in 

 their usual numerical proportion beneath the surface of the sands. 

 The average ratios between length, depth and thickness as deduced 

 from the average of 20 large and medium-sized individuals from 8 

 localities is 100 to 85-56 (depth) and 57"89 (thickness). 



Colour varieties. Of the various colour varieties, the three 

 principal (apart from the type), viz. impudica, castanea and aurora, 

 are all of very definite and stable colouration , and so well defined 



