GEOGRAPHICAL, DISTRIBUTION. HI 
The general effect of climate, irrespective of other 
influences, is undoubtedly a reduction of the number, 
both of genera and species, towards the north ; and a 
manifest diminution of the number, size, and perfection 
of individuals. Thus, while in the northern sections 
Glanclina, Helieina, Cyclostoma, and the polygyral forms 
of Helix entirely disappear, and only one of the native 
species of true Bulimus remains, the single genus Vltrina 
be fans to be observed. At the same time, although in 
O 
the southern sections the genera are more numerous, 
and only two northern genera, Tebennoplwrus and Vi- 
trina, are known to disappear, yet, the great genus 
Helix becomes less numerous in species, thus indicating 
that its focus is in the more temperate portion of the 
central region. This result however, is affected by 
elevation and other causes as well as latitude, species 
being continued on the Appalachian table lands, as far 
south as Georgia and Alabama, when they have already 
disappeared on either side in the same latitude. And 
while the northern species show a tendency gradually 
to run out, towards the south, their places are supplied 
to some extent by other forms. Thus, the polygyral 
Helices, which form a very distinct division of this 
genus, and by some have been thought to possess char- 
acters sufficiently marked to constitute a genus by them- 
selves, occurring but rarely in the latitude of the Ohio 
river, become more common towards the south, until, on 
the borders of the Gulf of Mexico, they exist in vast 
numbers, to the exclusion of nearly all the species, 
