INTRODUCTION. 



IT seems requisite for a clear apprehension of the following 

 pages, and for the forming of a just estimate of the authority to 

 which they may be entitled, that a few expositions should be here 

 given. 



No attempt has hitherto been made to give an account of all the 

 shells of any particular region on this continent. No book exists 

 in which we may find descriptions of any considerable proportion 

 of the whole number of the shells of the United States. They 

 are scattered through many volumes and periodicals, such as trav- 

 els, scientific journals, magazines, newspapers, &c. The works 

 of Say and Conrad are beautiful and accurate, as far as they go ; 

 and the Journal of the Academy of Natural Sciences at Philadel- 

 phia does indeed contain descriptions of very many shells, scat- 

 tered through its volumes ; but being unaccompanied by figures, 

 it is difficult oftentimes to identify the shell intended. Moreover, 

 all these works are rare and expensive ; and it is very difficult for 

 any one to collect together all the books requisite for the study of 

 the conchology of any one district of our country. Through 

 public libraries, however, and the liberality of individuals, I have 

 had access to every publication of which I have any knowledge, 

 that would be likely to aid me in my undertaking. The references 

 to descriptions and figures which I have made, will show to what 

 an extent these works have been consulted. They are not simply 

 quoted from some other authority, but almost every one of them 

 has been actually inspected. 



For the purpose of ascertaining the arrangement which recent 

 important progress in the study of the Mollusca would indicate, 



