GEOGRArHICAL DISTRIBUTION. J 21 



and crumbling limestone, "nhicli modifies the sandj soil, 

 the yegetation remaining unfavorable. This section is, 

 in its lower and more humid parts, the peculiar habitat 

 of the genus Grlandina, which has not been noticed 

 north of it. Here also exist in great numbers Helicina 

 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 eSect of a poor and sandy soil, much re- 

 duced in size. HC. lucuhrata, H. seleniiia, and H. mohili- 

 ana, so far as is known, are found here only ; so also is 

 Succinea campestris. Off-shoots from this section extend 

 into that next north of it, Helicina and one at least of the 

 folygyral helices having been noticed as far north as the 

 Ohio River. The species belonging to the section next 

 north are found more or less abundant 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. 15 



