OF SOUTHERN INDIA. 153 



Returning again to the cretaceous species, their determination also is to a very 

 great extent very unsatisfactory, though we already meet here vrith many character- 

 istic forms. We do not find as yet true Venus, but we meet with Mercenaria, a 

 group less numerous in the present seas than others ; some forms indicate Chione^ but 

 scarcely any resemble Cryptogramma. Species of the genus Cytherea are by far 

 more numerous than those of Venus. We meet some, though very few, which are 

 almost identical with recent forms of Callista, others very much resembling Cytherea 

 proper, very few somewhat allied to Tivela, but most of the species appear to 

 belong to Caryatis, which has a very wide geographical distribution, as already 

 stated. All these forms are especially prominently represented in the deposits of 

 the old continent, while in America we see that most of the species have been 

 referred to the present almost exclusively American Dione, though, as the sub-genus 

 has been restricted at the present, few belong to it, Callista and Caryatis besides 

 being apparently the most common. I mention this fact merely because it is an 

 additional proof of what I have already several times observed, that the present 

 geographical distribution of Molluscs was to some extent indicated already durino* 

 the cretaceous period. Such passing observations look very insignificant by them- 

 selves, but they are solid materials, out of which the palace for geological science 

 must be built ! As regards tertiary deposits I have little to say ; the species 

 gradually become very similar to the recent ones, and so is also their geographical 

 distribution. Caryatis during the eocene epoch is still most numerous, but then it 

 immediately passes (though scarcely in a larger number) into the present tropical 

 fauna, for we do not find it in the miocene (of Europe and America) in any number 

 nearly approximate to what occurs in the two last named epochs. 



The classification of the recent venerin^ noted above will be a great help in 

 obtaining an idea of the character of the cretaceous species, of which I shall give 

 a list at the end of the general remarks on the family. 



c. Suh^famili/,-^SUNETTIN^, 



The animal of the single recent genus referable to this sub-family is not yet 

 known, but I hope to be able before long to be in possession of one or two 

 of our common Indian species. The shell is ovately elongated, moderately com- 

 pressed, hinge with three or four compressed cardinal teeth and one lono* lunular 

 in each valve ; the posterior margin behind the beaks is peculiarly flexured, and 

 bent inside, forming a deep cavity, sometimes with corrugated sides and containino* 

 the ligament hidden, or almost hidden ; the lunula is linear ; pallial sinus distinct."" 



The general, in many respects Teltinoid, character of these shells appears to 

 make it probable that the animals will be somewhat distinct from those of the 

 true VENEEiN^. Deshayes suggested, as already referred to, that Sunetta may 

 belong to the Donacidm, but the form and arrangement of the hinge-teeth is 

 evidently that of the Venerid^, and that author himself classes ''the allied 

 arateloupia in the present family. The posterior areal excavation of the 

 8UMETTiN^ is Very characteristic for the recent as well as for the fossil species. 



2 p 



