GEOGRAPHICAL DISTRIBUTION. 121 
and crumbling limestone, which modifies the sandy soil, 
the vegetation remaining unfavorable. This section is, 
in its lower and more humid parts, the peculiar habitat 
of the genus Cflandina, which has not been noticed 
north of it. Here also exist in great numbers Hdieina 
orbiculata, and the polygyral species of Helix. These 
last are most abundant on the sea islands, and near the 
inlets from the sea. In the sandy plains is found the 
species of Helix called by Mr. Say fallax, by the 
influence of climate very numerous in individuals, but 
through the effect of a poor and sandy soil, much re- 
duced in size. H. lucubrata, H. selenina, and H. mobili- 
ana, so far as is known, are found here only ; so also is 
Suecinea campestris. Off-shoots from this section extend 
into that next north of it, Helicina and one at least of the 
polygyral helices having been noticed as far north as the 
Ohio Hiver. The species belonging to the section next 
north are found more or less abimdant where the two 
sections blend, and extend to a considerable distance 
into this section. 
In the southern part of this section, the State of 
Texas is included, the climate of which exceeds consid- 
erably in its mean temperature that of the parts of the 
same section on the Atlantic. After crossing the Mis- 
sissippi River, several species appear which either do 
not exist, or are very rare on the eastern side ; these 
increase in size and development as we proceed in a 
south-western direction, until they are found in the most 
mature condition in the western part of Texas. Promi- 
vol. i. 32 
