192 INTRODUCTION. 



ncnt among these are Bulimus dealhatus, wLicli is very 

 abundant, If. Buffoniana and H. herlanderiana. Helix 

 texasiana, a modification of the tridental forms, also ap- 

 pears, and seems to constitute a connecting link between 

 Helix tridentata and its varieties and the polygyral forms 

 so common throughout the rest of this section. Wliile 

 these new forms are developed, the common forms of 

 Helix se^Jtemvolva and auriculata diminish sensibly in 

 frequency, as we proceed towards the south-west along the 

 shores of the Mexican Gulf, and at length become very 

 rare or disappear entirely. Among these is Helix sep- 

 temvolva, a species more or less common in the low 

 lands of the whole coast from South Carolina to Louis- 

 iana inclusive. These circumstances indicate a tran- 

 sition to another fauna, and it is therefore reasonable to 

 suppose that, as regards the terrestrial moUusks, Mexico 

 is a distinct zoological region. 



Section 3. The Southern interior Section. In this 

 section is included the territory north of the preceding, 

 its western part lying between it and the Ohio River, 

 while its eastern part extends northerly, though not well 

 defined on its northern boundary, as far as North Caro- 

 lina or the southern part of Virginia. It is for the most 

 part broken into mountains or hills, is watered by large 

 and rapid rivers, and numerous streams and brooks, and 

 is covered with forests of deciduous trees. The cfimate, 

 geological structure, soil, and vegetation, are in general 

 favorable, and accordingly, both species and individuals 

 are numerous. As before mentioned, some of the spe- 



