12_J. 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. Vaijinulus, Glandina, Helicina, the larger 
species of Bulimus, and Gyolostoma, restrained by cli- 
matal influences, have disappeared ; but the majority of 
the species of Helix, Succinea, Pupa, 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. lueida, an introduced species, and Pupa 
badia, decora and gouldii, are perhaps the only ones. 
Section 5. The Northwestern 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. 
