yi PEEPACE. 



treats tlie fossil in connection with the recent forms upon the same basis, is 

 incomplete, and in many cases the relations of the fossil to recent types have 

 been entirely mistaken. So far, therefore, as the Indian species, and others 

 required for comparison, were concerned, a necessity arose for a revision of the 

 o-eueric groups to which they belonged. The references to the Cretaceous 

 Pelecypoda in general somewhat enlarged the scope of the undertaking, and it 

 appeared to me then that with a little additional information, the work might 

 prove a very useful compilation. I do not hesitate to say that I was also greatly 

 encoura""ed in carrying out this review by the many favorable opinions which had 

 reached me regarding similar reviews already given of some of the families of 



the Gastropoda. 



It must, however, be observed that this ' review of the genera' does not pretend 

 to be more than the word expresses. Even as far as completeness goes, the list 

 must be judged only with reference to the materials at my disposal. I wish I had 

 been able to call it a revision, but such would have required a great deal more 

 time a far superior anatomical and conchological knowledge, and essentially an 

 examination of typical species. Wherever these were accessible to me, I have 

 not omitted to make my characteristics as detailed as I believe to be requisite for 

 purposes of determination. But in a large number of cases my only resources 

 were the scattered notes published about the various genera, or the brief charac- 

 teristics, which, when based upon imperfect fossil specimens, are necessarily them- 

 selves imperfect, and, I am sorry to say, often only too much so. My object, 

 therefore, has necessarily been limited principally to giving a review of the present 

 state of our knowledge, leaving the completion of this to those who have more 

 opportunities of examining typical specimens. 



I hardly need to add that I will be greatly indebted for a notice of any 

 corrections or omissions which may be pointed out to me, and will give them 

 publicity with the least possible delay. 



The general classification into orders, and partially also that into families, &c., 

 differs considerably from that recorded in any other work on Conchology. I have 

 taken advantage of the examination of the animals of a great many species in 

 various groups, and this in connection with the study of the fossil forms has 

 suggested some alterations, which I have been anxious to introduce in the classi- 

 fication. 



Every one acquainted with this subject will be aware that it is most difficult 

 to arrange the Pelecypoda into well defined natural groups. The characters of the 



