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



(M. 10857/2) fromGwadar, Baluchistan, which have come under my 

 notice, show this species to be a most interesting link between 

 the species inhabiting the estuaries of continental India and those 

 from further east, notably the giant Japanese Meretrix called 

 " Hamaguri," M. lusona (Chemn.). The true Indian species, ex- 

 cept in the extreme forms of M. casta ovum, tend to assume short- 

 ness in the antero-posterior axis and incline either to a cordate or 

 to a sub-orbicular outline. In M. attenuata^ on the other hand, 

 the shell is very definitely produced posteriorly into an acute 

 angle, giving the shell a distinctly subcuneate outline, emphasized 

 by the straightness of the sharply declivous margin between the 

 hinge and the posterior extremity. In this elongation and straight- 

 ness of the upper posterior margin M . attenuata approaches the 

 large Japanese species, which however is considerably more elong- 

 ated. These two species agree in nearly every detail of the hinge 

 region ; in both the anterior tooth is inserted on a shelf set at an 

 angle of about 45 degrees to the ventral edge of the main hinge 

 plate No such great obliquity occurs either in M. casta or M. 

 meretrix. Further, although the anterior cardinal tooth both in 

 M. attenuata and the Japanese species is marked by a very slight 

 striation on the apex, this is so obscure and weak as not to be ob- 

 servable except under a lens of considerable power ; there is no 

 approach to the sub-bifid form seen in M. meretrix. 



This species is so rare in collections that the three specimens 

 possessed by the Indian Museum deserve careful attention, especi- 

 ally as Dunker and Romer appear to have based their diagnosis 

 upon a single specimen of unknown origin. 



The smaller of the two Nicobar specimens agrees in almost all 

 particulars with the described form and is almost of the same 

 size, its dimensions being 53X43X25 mm. against Bunker's 59 X 

 40^X25 mm. The colouration is almost identical with that of 

 Bunker's and Romer's figures, the shell being covered with con- 

 centric bands or zones of broken chevron markings of a chestnut 

 tint that remind one of the graphica form of var. zonaria in M. 

 meretrix. The periostracum is thin, somewhat dull and olivaceous 

 yellow. Internally the shell is white with violet staining along 

 the posterior declivous margin. The hinge plates are malformed, 

 due to the fusion of the anterior and median cardinals of the 

 right value. Apart from this, in one important point the hinge 

 differs from the type description ; Bunker states the nymphae are 

 " tenerrime granulatae nee denticulatae ," which Romer varies b}^ 

 saying " tenerrime granulatae nee transversim sulcatae." Now in 

 this smaller Nicobar specimen the nymphae are quite distinctly 

 marked by closely-set transverse ridges, each of which does how- 

 ever show signs of being composed of a row of fine granula- 

 tions 



The larger of the Nicobar specimens and the single one from 

 Gwadar in Baluchistan are quite differently coloured, but in all 

 essential particulars otherwise agree. In both, the valves are 

 virtually bereft of markings and are covered with a thin and pale 



