EXPLANATION OF PLATE V. 



Meretrix meretrix (Linn.). Type. 



Fig. 13. — An adult specimen from Tuticorin, Observe the faint 

 spotting on the umbo, the absence of rays and the indistinct 

 margin of the dark coloured posterior margin or " vulva." 



Meretrix meretrix var. impudica (Chemn.). 



Figs. 14, 15, 16 and 17. — These young shells show the rayed and 

 zoned colouring characteristic of this variety in the very 

 young condition. 



Figs. 14 and 15 (Chilka Lake) were identified with M.ovum by Preston, 

 but comparison with fig-. 39, pi. vii, shows clearly that they develop into 

 typical M. meretrix var. impudica. 



Figs. 16 and 17 are from Tuticorin. The sharply margined dark vulva 

 is clearly shown in all four figures. 



Fig, 18. — A shell showing colouring hybrid between var. morphina 

 and var. impudica. LocaUty, Chilka Lake. 



Meretrix casta var. satparaensis (Preston). 



Figs. 19, 20 and 21. — Three typical left valves from Korampalam 

 shell-pits, Tuticorin. Note the emphatic *' hooding" of the 

 umbo in all, and the gradation of figs. 19 and 20 into the 

 type of M. casta as seen in figs. 22 and 23. 



Meretrix casta (Chemn.). Type. 



Fig. 22. — A large specimen from Pulicat Lake with elongated pos- 

 terior suggesting transition to the var. ovum form seen in 

 fig. 24. 



Meretrix casta var. ovum (Hanley). 



P'iG. 23. — A stout shortened form from the mouth of the Baliapa- 

 tam River, North Malabar. It approaches closely in outline 

 to the central type as seen in Chilka and Pulicat Lakes. 



Figs. 24, 25 and 26. — Three forms of this variety exhibiting the 

 gradual lengthening of the shell, and the ultimate assumption 

 of an Unio-like outline, very different from the cordate form 

 of fig. 23. All from Tellicherry. 



All the figures are natural size. 



