PEEFACE. 



In introducing tlie third volume of the South Indian cretaceous fossils, con- 

 taining a monograph of the Pelecypoda,* I have but few remarks to make. 



A general historical sketch, with some notes on the different classes of MoUusca, 

 was given in the introduction to the Gastropoda (Vol. II of Cretaceous Eauna, 

 Palseontologia Indica). I have nothing important to add to those notes. f 



The monograph of the Pelecypoda itself, strictly speaking, consists of three 

 divisions : 1, a review of all the known (to me) recent and fossil genera ; 2, an 

 enumeration of all the cretaceous species ; and 3, descriptions, accompanied with 

 figures, of the species known to occur in the cretaceous deposits of the Trichinopoly 

 and South Ai-cot Districts of Southern India. A few words on each of these 

 divisions will suf&ce in the way of explanation. 



1. It may perhaps be thought presumptuous on my part to undertake so 

 important a work as to review aU the known fossil and recent genera of Pelecypoda. 

 No one will hesitate to admit that a work of this kind is most difficult, and I can 

 only say that I have discharged the dvity as best I could with the materials at my 

 disposal, both in respect of specimens and literature. 



The reasons why I undertook the work at all were several. When I entered 

 upon the examination of the South Indian Cretaceous Pelecypoda, I found that 

 nearly all the principal groups in this class of Mollusca were represented among 

 them. The importance of a correct generic determination, and the necessity for a 

 comparison of several of the species with others, known from different countries, 

 were obvious; they were in fact rendered essential by the materials of our 

 collection. But the difficulty then arose upon what basis the work should be 

 executed. H. and A. Adams' "Genera of recent Mollusca" appeared to melhe 

 only work which I could follow, as I have already had occasion to observe 

 in my introduction to the Gastropoda. Chenu's Manual, the only Avork which 



* Conchifera, Bivalvia, Zamellibranc/iiata, ^c, of other authors, see further on p. 8 and seq. 



t Observations on classification of Mollusca, such as those by Prof. Hincks published in the Canadian Journal 

 of Industry, Science, and Ai-ts for 1867, have very little influence upon the advance of the subject. Some others I will 

 notice when occasion offers itself in working out the different classes. 



