GEOGRAPHICAL DISTRIBUTION. 113 
ations, we have not been able to perceive any ap- 
preciable differences in the influence which they exert, 
but from all of them it is less favorable. The coal 
formations have been said to be nearly as favorable to 
their multiplication as calcareous strata. We are not 
prepared to deny this entirely, but certainly, in every 
part of the coal-field of the eastern part of Ohio which 
we have visited where limestone is wanting, these ani- 
mals are not more numerous than in the primary region 
of Vermont, other circumstances in both being appar- 
ently equal. The only inference therefore which seems 
to be justified is, that the presence of calcareous matter 
in the soil produces a positively favorable effect on the 
increase of the terrestrial mollusks ; the effect of the 
other geological formations being only negative, and, so 
far as observation goes in this country, not definite. 
Vegetation. A vegetation composed entirely of coni- 
ferous trees produces a positively unfavorable influence. 
Hence, in our pine forests, these animals are always 
comparatively rare, and in the northern primary sections 
may be said to be entirely wanting. In the Southern 
States, where extensive pine forests prevail, but with a 
favorable climate and a calcareous soil, neither species 
nor individuals are numerous, and the latter are reduced 
in size. It seems necessary, indeed, that deciduous 
trees and plants should predominate, in order to their 
considerable multiplication and extension. 
Humidity and Dryness. Humidity is one of the 
conditions necessary to their existence, and therefore it 
vol. I. 30 
