igiy.] J. H OK NELL : Indian species of Meretrix. 165 



straw-coloured periostracum. Anteriorly there is a con,siderable 

 amount of extraneous black staining, and on the umbones of the 

 Gwadar shell are several imperfect zones of chevron marks. In- 

 ternally the posterior declivous margin is characteristically stained 

 violet. 



In both, the nymphae conform more closely with Bunker's 

 and Romer's descriptions than does the smaller Nicobar speci- 

 men ; the transverse character of the striae has become obscured 

 and the granulations are apparently irregularly disposed ; only 

 here and there can a faint suggestion of transverse disposition be 

 seen. The second cardinal tooth of the left valve of the Gwadar 

 shell is abnormally weak, and it is noteworthy that out of 

 three specimens the cardinal tooth of two depart from the 

 normal. 



In all three specimens the pallial sinus is similar. It is 

 strongly marked, deep and almost semicircular. The ventral horn 

 ends in an acute and downwardly turned point approaching closely 

 the form seen in M . lusoria but more inclined ventrally. 



From the above it is obvious that both Bunker's and Romer's 

 descriptions require amendment in two points, namely, sculpturing 

 of the nymphae and the external colouration of the valves. From 

 what I see in this species, reinforced by examination of a very 

 large series of M. meretrix as also of several large M. lusoria^ I am 

 able to say that the sculpturing of the nymphae, in those species of 

 Meretrix which attain relatively large size, undergoes distinct 

 degeneration with increase of size and age. In young shells the 

 nymphae are marked by coarse transverse ridges, very distinct and 

 set comparatively widely apart. With advance in age the ridges 

 decrease in prominence while increasing rapidly in number and 

 become more closely set. Finally the ridges begin to break up 

 into rows of granulations, and in the final stage, marking old age, 

 the parallel arrangement in rows of the granulations may even 

 disappear entirely, and nothing remain except an area closely set 

 with extremely fine and exceedingly numerous minute granula- 

 tions without pattern or order. 



As there appear to be two distinct colour varieties of this spe- 

 cies I propose to separate the nearly unicoloured form under the 

 name fiava diagnosed as follows : — 



Var. flava, var. nov. 

 (Plate VII, figs. 41 and 42.) 



Similar to the type in all particulars except in the external 

 colouration of the valves, the zones of zig-zag markings characteris- 

 tic of the latter being suppressed except sometimes partially upon 

 the umbones ; general tint pale straw colour due to the tint of the 

 thin investing periostracum. 



Habitat. — Nicobar Islands and Gwadar, Baluchistan (Indian 

 Museum collection). 



