[v.] 



sliould first be induced to collect and name, afterwards to 

 arrange his collections upon the study of their habits and 

 affinities; and that naturalists would become ten times more 

 numerous were they not appalled at the outset by the im- 

 mensity of the subject, which afterwards proves its greatest 

 charm. 



There are, besides, many persons whose leisure or tastes do 

 not permit them ever to become, strictly speaking, naturalists, 

 but who, nevertheless, if furnished with proper guides, would 

 gladly devote a few hours occasionally to the pursuit of some 

 branch of natural history, although they would never engage 

 in it to such extent as to make it a study; and we feel that 

 this large class of persons should be encouraged to do as 

 much as possible for the benefit of science by collecting the 

 material necessary for the researches of the educated naturalist. 



Whenever good specimens could not be readily obtained 

 for figuring, recourse has been had to previously published 

 figures, and my acknowledgments are due to the works of 

 Messrs. Binney, Bland, Lea and others, for the opportunity 

 afforded by them for completing the illustrations. Very many 

 of the shells are figured for the first time, as this is the first 

 complete monography published since that of Dr. Binney. 



The large number of new species described since that period 

 have been 'classified in accordance with their affinities with 

 those previously characterized. 



At the close of the work will be found a list of American 

 works on terrestrial species, and also a systematic list of the 

 species and their principal synonyms. This has been deemed 

 far preferable in the present instance (being more in accordance 

 with the plan and purpose which I have endeavored to explain), 

 to the usual method of prefixing the synonymy to the descrip- 

 tion of each species. 



GEOEGE W. TKYON, Je, 



625 Harket Street, 

 Philadelphia, July 1st, 1866. 



