166 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 Vaginulus, Crlandina, Cylindrella and Helicina, 

 which are believed to be 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, withm which 

 they all prevail. Crlandina, 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 Cyclostoma are most numerous in the Antilles, 

 from which most probably our species, w\.\h. one excep- 

 tion, were derived. The sub-genus Helicodonta of 

 Ferussac, 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. 



