158 INTRODUCTION. 
ders. Others are met with only adjacent to the sea, 
or rather not very far removed into the interior, as 
Helix cellaria, and Limax agrestis, which in mam- 
localities are very common. The extreme distance 
from the sea-shore at which we have hitherto noticed 
them, has not exceeded one hundred miles. Others 
again, as Limax variegatus, occur in the cities and 
other limited localities separated by considerable dis- 
tances from each other, whither they have been trans- 
planted by accident ; and yet another class are confined 
to a single locality, beyond which they have not yet 
advanced, as Bvlimus decollatus in gardens in Charles- 
ton, S. C, and Helix lueida in the vicinity of Albany, 
N. Y., where they have probably been introduced in 
packages of imported plants. As all these are probably 
destined to a still farther extension, the present range of 
each is important as a point of comparison in future 
investigations. 
The annexed table exhibits a list of foreign species, 
which have been stated by authors to occur in the Uni- 
ted States, or the countries adjacent, with an indication 
of the present condition of those now existing. It is 
quite probable that some of the species, said to be ex- 
tinct, have never in fact occurred here ; and that others, 
which now seem to be identical with foreign species, may 
be found essentially different when examined more accu- 
rately, and the difference may become still more obvious 
on a careful comparison of the animals. 
Foreign species now first ascertained to be found in 
the United States, are not included in the table. • 
