LETTER OF TKAXSMITTAL. 



SIR: In accordance with your request that I should prepare an arti- 

 cle for your annual report upon a subject which has for some years been 

 engaging my attention, I have the honor to submit herewith the follow- 

 ing, which I have entitled U A Keview of the Non-Marine Fossil Mol- 

 lusca of North America." 



In the preparation of this article I have endeavored as fully as prac- 

 ticable to follow your suggestion, that it should be as free from techni- 

 cal forms and methods as the nature of the subject will allow. I have 

 therefore endeavored to address the general reader rather than the spe- 

 cial investigator,* but I have given copious references in the form of 

 foot-notes, so that those who wish to pursue the subject further may 

 readily refer to nearly all that has Been published upon it in America. 



Your recognition of the fact that there is a natural and growing 

 desire on the part of intelligent readers to know something of the geo- 

 logical history of the predecessors of the animals with which they are 

 more or less familiar, or which are frequently referred to in the books 

 they read, would be expected by those who are familiar with the graphic 

 style of your own writings; and your wish to gratify that desire is too 

 obviously correct to make any explanation or apology proper on the 

 part of a specialist who may be called upon to communicate with the 

 public in the form proposed by you. 



In the selection of a subject, I have chosen one which, although 

 primarily based upon molluscan species which are all extinct, embraces 

 the consideration of, or reference to, living forms, congeners of those 

 which have ceased to exist, that are scattered over all parts of the 

 country. It is, therefore, a subject which a far greater number of per- 

 sons will find of ready application within their every day experience 

 than many others which the wide range of paleontology might furnish*. 

 In illustrating this subject on the accompanying plates, I have in each 

 case selected such figures (which are in part copies of illustrations 

 already published by various authors, but largely newly drawn from the 

 type-specimens of the various species) as would exhibit the form, and 

 such features of the objects, respectively, as strike the eye of the ordinary 

 observer, omitting, in many cases at least, those details of structure 

 which more especially engage the attention of the special investigator. 



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