1(3(3 INTRODUCTION. 
a few deductions from some of the foreign lists, main- 
tains nearly an equal representation. The most consid- 
erable differences, however, indicated by the comparison, 
arise from the introduction into our catalogue of the 
genera Vaginvlus, Grlandina, Cylindrella and ILUcina, 
which are believed to he entirely wanting in Europe, 
and the very distinct section of Helix, which includes the 
polygyral species. All these are due to our near ap- 
proach to the tropical parts of our continent, within which 
they all prevail. Grlandina, it is believed, is peculiar 
to the coasts and islands of the Gulf of Mexico, the 
alluvial regions north and south of it, and the larger 
West Indian Islands; while Helicina and Cylindrella, as 
well as Cychstoma are most numerous in the Antilles, 
from which most probably our species, with one excep- 
tion, were derived. The sub-genus Helicodonta of 
FeVussac, which is numerous in species, as we shall 
hereafter have occasion to observe, is also characteristic 
of the American continent and islands, but is not, like 
the preceding, more predominant near the equator than 
elsewhere. We shall notice the peculiarities which dis- 
tinguish our native species, and the particulars in which 
they differ from those of other countries, in our general 
remarks upon each genus. We give here a comparative 
table showing the number of species of the several 
genera of terrestrial mollusks in the United States, and 
in various sections of Europe. It is derived from au- 
thentic catalogues of local authors, and exhibits forcibly 
our numerical deficiencies in species. A single local 
catalogue of the North of Africa is added. 
