i66 Records of the Indian Museum. [Vol. XIII, 



Dimensions. — The larger Nicobar shell, 63X50 mm.; Gwadar 

 shell, 64X52X35 mm., the latter giving the ratio of 100 to 8i'25 

 to 54-69. 



3. Meretrix casta (Chemnitz). 



(Plate V, fig. 22 ; plate VI, figs. 30-33.) 



1782. Venus casta, Chemnit/, Couch. Cab., Vol. \'I, p. 349, pi. 2,?<- 

 1835. CytJierea casta, Lamarck, Auim. sans. Vert., 2nd ed., Vol. \'I, p, 301. 

 184'^. ,, ovuju, Hanley, Pj'oc. Zool. Soc. Loudon, 1845, p. 21. 



i86q. Meretrix casta, Romer, MojwgrapJiie der Molluskeugattiiug I'euns, 

 Linne, Band I, subgenus Cvtiierea, p. 31, pi. xii, 

 fig. 2. 

 ovum, Romer, ibid., p. 38, pi. xi, fig. 4. 

 e.xilis, Romer, ibid., p. 35, pi. xi, fig. 3. 

 IQ14. Corbicnla ( Velorita) satparaensis, Preston, Rec. I)id. Mus., X, p. 306, 



figs. 22 and 22a, p. 308. 

 IQIS- Meretrix casta, Preston, ibid., XI, p. 300 (not his large valve 67 X y.v 

 mm. which is M. meretrix). 

 ,, ovum, Preston, ibid., p. 300. 



morphiua, Preston, ibid., p. 300. 

 1016. ,, casta, .\nnandale and Kemp, Mem. lud. Mus., V, p. 351 



(not their M. ovum which is M. meretrix impudica 

 juv.). 



This species is exceedingly variable and ignorance of this fact 

 has caused great confusion over its nomenclature ; in M. meretrix 

 the variation is limited largely to the colour design of the valves ; 

 in M . casta, the form of the shell and the proportionate develop- 

 ment of the hinge elements are subject to a wide range of varia- 

 tion, apart from and in addition to much diversity in the surface 

 colour scheme of the valves. 



The form desciibed by Chemnitz and by I.amarck under the 

 name casta may be considered the type of the species, as the 

 latter's definition accurately summarises the characters of the pre- 

 dominant form found in east coast backwaters and estuaries from 

 the Chilka Lake to Tuticorin. According to Lamarck the shell is 

 " cordato-rotundata, gibba, crassa, alba; pube anoque ovatis, 

 convexis, glaucescentibus ; intus violaceo maculata." Shells 

 agreeing with this description never have well developed radial 

 bands, but on some medium-sized individuals from the Chilka Lake 

 (M. 10589/2 A and B) two short unmistakable rays can be made 

 out on each umbo, and in one case I can trace one of these bands 

 continued as a faint dusky clouding for a considerable distance 

 towards the margin. In variety ovum from Malabar the rays are 

 less frequently suppressed (pi. IV, figs. 3 and 4). Hanley's Cy- 

 therea ovum is a subequilateral variation, whilst Romer's M. exilis 

 is nothing but a young stage of the same variety. 



The distributional range of the type form is limited to the 

 east coast of India, where it is found, often in densely stocked 

 beds, in the majority of the backwaters and estuarine channels 

 from Orissa to Cape Comorin. The sub-species ovum on the other 

 hand is not found except in occasional and rare individual cases 

 within these limits, but is exceedingly common upon the west 



