OF SOUTHEEN INDIA. 201 



American CorhictUce, Cyrence, and others possess a pallial sinus, while the species 

 from other countries have the pallial impression simple. The same author suggests 

 that fm'ther anatomical examinations, upon which he is at present engaged, may 

 lead to the discovery of characters which could form the basis of generic distinction. 

 As far as the pallial sinus is concerned, I cannot but doubt this, although I have had 

 lao opportunity of examining the animals of any of the American species. Cyrena 

 Ceylanica, Siimatrana, and Bengalensis all possess a more or less distinct pallial 

 sinus, though I readily admit that it is not so well developed and so deep as in 

 C. Carolinensis or Salmacida, but there can be no mistake as to its existence in 

 either of the cases. All the Indian and Java species of Corbicula (about twelve) which 

 I have examined equally show the posterior end of the pallial impression either 

 truncated or distinctly and usually broadly insinuated. I state this merely because 

 I have from my own materials not been able to detect any generic distinction 

 between the American and the Eastern Corhiculce and Cyrenai, but by no means 

 with an object to anticipate, or differ from, any conclusions to which Mr. Prime's 

 large materials may lead. His results will no doubt be more weighty than those 

 derived from the comparison of a few species. 



In addition to the well known six recent genera of the Ctrenid^, I will quote 

 here only two fossil ones, Diodus, Gabb, based upon a cretaceous species from 

 California, and Isodoma, Desh., based upon an eocene shell from the Paris basin. 

 The former represents the group of genera with three cardinal teeth in each valve, 

 and the latter the one with only two cardinal teeth. 



1. Cyrena, Lam., 1806. Shell sub-cordiform, robust, moderately inflated, 

 hinge with three sub-parallel, more or less bifid cardinal teeth, one strong, smooth, 

 or very minutely and partially transversally striated* lateral tooth on either side ; 

 pallial sinus short, more or less distinctly indicated, sometimes nearly obsolete. 



2. i)wc??<s, Gabb, 1868, (Pal. Calif, ii, p. 242; Cyprmella, Gabb, olim). 

 Shell equivalve, sub-cordiform ; hinge with three diverging, (simple ?) cardinal 

 teeth and one anterior and one posterior lateral smooth tooth in each valve ; pallial 

 sinus shallow. 



This is proposed for a doubtfiilly cretaceous species, D. tenuis, Gabb, from 

 California. It is difficult to distinguish this suggested new genus from Cyrena, 

 except that the three cardinal teeth of the right valve are in Gabb's figure 

 thinner and more distinctly diverging from one point than is usually the case 

 in recent species of Cyrena. Should this position of the hinge-teeth prove to 

 be of any value as compared with the last named genus, Zittel's Cyr. solUarla 

 may also be referred to Diodus, but I hardly think that we can look upon this 

 division in a more than sub-generic sense, perhaps scarcely in that. 



3. Velorita, Gray, 1834. Shell cordate, inflated, posteriorly somewhat 

 attenuated and produced, robust, lateral teeth very finely striated, somewhat 

 elongated, the anterior lateral is very close to and almost touching the most 



* In Cyrena Bengalensis for instance. 



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