PEETACE. 



In the present Monograph of American Corbiculad^, pre- 

 pared at the request of the Smithsonian Institution, it is proposed 

 to show the present state of our knowledge of the species, both 

 recent and fossil, which inhabit North and South America. It 

 contains descriptions of all the genera of the family, whether 

 represented on this continent or not, descriptions of the species 

 found in North and South America, notices of their geographical 

 range, references to the collections in which authentic types of 

 many of the species are known to exist, and comparisons of the 

 different species with others of the same genus, indigenous and 

 foreign. 



I have been able to identify to my entire satisfaction very nearly 

 all the species described as from America, and the instances in 

 which I have not been successful, are duly noted in the text 

 accompanying the description of the species. 



I am aware of the fact that some of the genera adopted in these 

 pages, based chiefly upon characters drawn from the shell alone, 

 ought not to be retained with their present limits ; nevertheless 

 our knowledge of the soft parts of the species of this family is 

 still so very imperfect that no other course is open to me but to 

 preserve for the present the genera as I find them, however de- 

 fective they may actually be. 



It will soon be necessary, in order to keep pace with other 

 departments of natural history, to introduce some modifications 

 in the limits of the genera of the Corbiculadee, but no really satis- 

 factory or permanent result will be attained until a careful exami- 

 nation of the soft parts shall have been made. 



I am at present engaged upon a new arrangement of the genera 

 of the Corhiculadse, based upon characters drawn from the soft 

 parts and from the shell, and in order to call attention to this 



( iii ) 



