GEOGRAPHICAL DISTRIBUTION. W^ 



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 mdividuals 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. 14 



