124 INTRODUCTION. 



operating within this section are of the most favorable 

 kind, except the climate, which, though considerably 

 severe in winter, is in the northern part meliorated by 

 the presence of the great body of the waters of the 

 lakes. Soil, vegetation, and geological structure com- 

 bine with the climate to produce, in the southern part of 

 this section, as well as in the northern part of the pre- 

 ceding, a greater amount of circumstances and condi- 

 tions propitious to their existence and increase, than exist 

 elsewhere. Vaginulus, Glandina, JTelicina, the larger 

 species of BuUmus, and Ci/dostoma, restrained by cli- 

 matal influences, have disappeared ; but the majority of 

 the species of Helix, Suceinea, Piqja, and Vertigo flour- 

 ish in great numbers, and all the naked slugs, both 

 native and introduced, with the exception of Vaginulus, 

 are found. It would seem to be the great central focus 

 of all these genera, from which they radiate into the 

 other sections. Very few species have as yet been 

 noticed ; H. lucida, an introduced species, and Pupa 

 badia, decora and gouldii, are perhaps the only ones. 



Section 5. The North-ivestern Section. The coun- 

 try west and north-west of the Great Lakes and extend- 

 ing to the 49th degree of north latitude. This section 

 is described to be " literally a wilderness of lakes, islands, 

 and peninsulas ; a mazy waste, so inhospitable and irre- 

 claimable, as to mock the art and enterprise of man, and 

 bid defiance to his industry." It is characterized as the 

 coldest climate of the United States ; the winters being 

 long and severe, the change from summer to winter being 

 rapid, and the extremes of temperature very great. 



