164 mTRODUCTION. 



The whole number of species hitherto ascertained, 

 within the bounds of the United States east of the Mis- 

 sissippi River, excluding species found elsewhere also, is 

 ninetj-nine. 



The first reflection that arises, on examining this table, 

 is, the comparatively small number of species which it 

 exhibits. The number inhabiting the corresponding cli- 

 matal parts of Europe, of equal territorial area, is more 

 than twice as great ; and the limited extent of the British 

 Islands affords at least an equal number. This in- 

 equality of numbers may perhaps be partly explained by 

 the supposition, that many species remain yet undiscov- 

 ered ; but, considering the attention which has already 

 been given to our conchology, we can hardly expect that 

 the whole difference should be made up by future re- 

 searches. A further approximation of numbers may 

 also be gained by cutting off some of the nominal species 

 of European Limaces, which seem to be established on 

 very slight external characters, and will hardly maintain 

 their places ; but when this is done, the difference 

 remaining will still be very considerable. We confess 

 that we do not perceive the means of reconciling the 

 disparity of distribution, which strikes us as the more 

 remarkable, because the species live in this country 

 more in their natural condition than they can do in the 

 cultivated parts of Europe, and because a large portion of 

 our territory seems to be, by its climate and geological 

 structure, and other physical circumstances, peculiarly 

 adapted to their growth and increase. The only circum- 

 stance which we perceive, wherein the European local- 



