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 Limnx agrestis, which in many 

 localities are very common. The extreme distance 

 from the sea-shore at which we have hitherto noticed 

 them, has not exceeded one hmidred miles. Others 

 again, as Liniax 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 Bidimus decollatus m gardens in Charles- 

 ton, S. C, and Helix lucida 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 further 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 existmg. 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. 



