and their Geographical Distribution. 489 
commerce has introduced and which have not yet 
travelled away from the sea-shore. Helix variabilis 
and pisana, and Bulimus acutus, however, which 
Mr. Forbes also sets down ‘as introduced to this 
continent, have not yet been found, so faras I am 
able to learn, while he does not mention Bulimus 
decollatus, which is not infrequent in South Carolina 
and Georgia. Alasmodon margaritifera and A. ar- 
cuata, are also regarded by many as identical. But 
all the specimens of the former which I have had an 
opportunity to examine, differ very essentially from 
any specimens of our shell which I have ever seen. — 
Indeed, I am not yet satisfied that any fresh water 
mollusk is common to both sides of the Atlantic. 
Animals living on territories widely separated by salt 
water, are always found to: be different, unless the 
means of intercommunication. are obvious, or at least 
possible. They appear to have been created distinct, 
and ever to remain so, unless — by design or 
accidental transfer. 
About twenty of the larger species may be regard- 
ed as intermediate between the two continents, as 
they are most abundantly found about Newfound- 
land and the Grand Banks, where they are an abun- 
dant source of food tó the eh which are caught 
there. 
Several thet species have been found in other 
States on the immediate borders of this State, which 
will, undoubtedly, be found within its limits ere 
long. 
The iédurufihiéel distribution of shells i is a subject 
which is daily gaining additional interest, and assum- 
