PREFACE. 21 
The work contains descriptions of all the air- 
breathing niollusks of the United States, living 
upon the land, including both the aboriginal and 
those foreign species which, having been from time 
to time introduced, have now become naturalized 
to the soil ; and of such species belonging to the 
countries adjacent as are known to the author. 
Our knowledge of these animals is brought down 
to the date of publication ; and it is hoped that 
the work will be found to be as complete as any 
record of an actively progressive science can be. 
In its preparation, all the American authorities 
have been consulted, and for the benefit of Euro- 
pean naturalists, some of whom appear to be 
inexcusably ignorant or wilfully neglectful of 
what has been done for Natural History in this 
country, a catalogue of American authors, whose 
works have been used in compiling these pages, 
is given ; a catalogue of the foreign works, 
referred to, is also added. A full synonomy of 
the native species is attempted, and it will pro- 
bably be found to be pretty complete ; but for 
the introduced species a few references only to 
well known works are given. 
Great care has been taken to investigate the 
