PREFACE. 



THE reader of the Introduction to this work, and of Dr. Gould's 

 Introduction to his great work on the Shells of the United 

 States' Exploring Expedition, may be struck with the coinci- 

 dence of opinions relating to the geographical distribution of 

 species, and to errors in the statement of their habitats and 

 distinctive characters. The coincidence is so exact, that it 

 might naturally be supposed that these opinions originated 

 in a single source. Such was their origin ; but that source was 

 the book of nature. It was not until after both Introductions 

 had been written, that opportunities occurred for a free inter- 

 change of views with Dr. Gould. It is therefore with the 

 liveliest satisfaction that we derive assurance of their correctness, 

 both from the extent and accuracy of Dr. Gould's knowledge 

 and also from the coincidence of the results of independent 

 investigations. 



With much pleasure I acknowledge my great obligations to 

 JOHN H. REDFIELD, Esq., for his assistance in carrying this little 

 work through the press. The delay in printing the first three 

 or four sheets was so great, that it became necessary to employ 

 another printer and to work off the sheets without revision by 

 the author. Owing, however, to the accuracy of this printer, 

 and to the care of Mr. Redfield, very few, if any, errors worthy 

 of notice appear, which are not chargeable to the manuscript. 



