152 INTRODUCTION. 
extend themselves gradually in the country. Of these, 
Liitiiu ■ iii/rcstis, and L.variegatus, Selix aspersa, II. eel- 
laria, II. hrrtensis, II. lucida, and //. pulcJiella, and 
Bulimus decollatus, B. zebra, B.fasciatus, B. virgida- 
tus and B. lulricus, are examples. Others strive in 
vain to establish themselves ; the climate or some other 
cause is fatal to them, and after some increase, they 
diminish and become extinct. Of these, Helix ru moralis, 
virgata, pisana, and lactea, and Bulimus acutm, are 
recorded as having been noticed in the United States 
at different times ; and it is only by adopting the sup- 
position of their subsequent extinction, that we can give 
credit to these observations, for it is nearly certain, that 
none of them now exist within our territory. 
Oceanic currents also aid in bringing to our shores 
foreign species, and have been the means of introducing 
and naturalizing them. The Gulf Stream is a promi- 
nent example of this. This great body of water, flow- 
ing .from the Gulf of Mexico into the Atlantic, passes 
between the Peninsula of Florida and the Island of 
Cuba, and after turning the southern point of Florida, 
3weeps along its eastern shore. It is sometimes driven 
close to the northern coast of Cuba, and sometimes 
forced much further north, according to the direction 
and force of the wind. Various counter currents, due 
also to the influence of the wind, diverge from the main 
stream, among which is noticed a current which, after a 
northerly wind has prevailed fir several days, sets in a 
south-westerly direction near to the Florida Reef. The 
