OF SOUTHERN INDIA. 203 



valves close the teetli are placed parallel or nearly so beside each other; in 

 Sphcerlum, however, the cardinal teeth are tubercular, or of a columnar shape, placed 

 obliquely towards each other, and when the valves close the axes in which they 

 are placed partially cross each other, though the position is not so distinctly deve- 

 loped as in the Cardiid^, but the passage from the present to the next family 

 is distinctly indicated by this arrangement. Sometimes the anterior cardinal tooth 

 of the left valve is in Sphcermm obsolete, at other times it is replaced by a very low 

 indistinctly bituberculated ridge. 



In a geological point of view the Cyrenibje are not very important, for it is 

 not often that we meet with fossil species belonging to them, a fact due to the 

 rarity of fresh-water deposits. Nothing positive is known of their occurrence in 

 carboniferous rocks, though there are various species described which in form 

 greatly resemble some VelorUcB and Cyrence. From the middle Jurassic also a few 

 species have been recorded by Forbes and others, but their determination is unsa- 

 tisfactory, though they to all appearance belong to the present family. The "Wealden, 

 which may be considered rather as the close of the Jurassic than the beginning of 

 the cretaceous period, contains a tolerably large fauna of Ctrenid^. Most of the 

 sijecies described by Sowerby (Trans. Geol. Soc. and Min. Conch.), Homer and 

 Dunker (Norddeutsch. "Wealdenb., &c.), and others under the generic name Ci/reiia 

 appear to belong to the Coi'hicula type, having the lateral teeth much elongated, 

 and thus indicating a true fresh-water fauna. The species require a critical examin- 

 ation, for if Bunker's statement, — that they all possess only two cardinal teeth 

 in each valve — is unexceptionally correct they must form a separate genus, dis- 

 tinct from Corbicnla, because they do not agree, on account of the length of the 

 lateral teeth, with either Fisidinm or SphcBriiim ; these two last genera also seem to 

 be represented already in the Wealden deposits, though the generic determination 

 can by no means be regarded as satisfactory. With regard to the Wealden so-called 

 Cyrence, it is to be remembered that in all the recent Corbiculce the most anterior 

 cardinal tooth of the right valve and the posterior of the left one are very small 

 and occasionally nearly obsolete, those teeth being only indicated by a very slight 

 swelling of the termination of the lateral teeth. It seems probable'that at least 

 in some of the Wealden species, as Cyrena Seysii, Dkr., C. elliptica, Dkr., 

 C. ovalis, Dkr., and others, such is really the case, for the position of the remaining 

 cardinal teeth almost indicates it ; we would, therefore, have in those species the 

 first known representatives of Corbicnla. Unless, however, this point has been 

 satisfactorily settled, it would be premature to suggest any new generic changes. 

 Species like Cijrena caudata, and a few others, would seem to indicate also the 

 occurrence of Velorita in Wealden rocks. 



From the cretaceous deposits we have only a few species upon record as 

 Diodus tenuis, Gabb. (antea p. 201), Cyrena solitaria, Zittel, (Denksch. Acad., Wien, 

 xxiv, pt. ii, p. 133), and Cy. cretacea, Drescher, (Zeitsch. Deutsch. Geol. 

 Gesellsch., XV, p. 345). The first two may belong to one and the same genus, or 



