1-22 
INTRODUCTION. 
nent among these' are Bulimus dealbatus, which is very 
abundant, H. Buffoniana and IT. berlandt nana. 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. While 
these new forms are developed, the common forms of 
Helix septemvolva 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 mollusks, Mexico 
is a distinct zoological region. 
Section 3. The Southern interior Section. Iu this 
section is included the territory north of the preceding, 
its western part lying between it and the Ohio Elver, 
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 
ia covered with forests of deciduous trees. The climate, 
geological structure, soil, and vegetation, are in general 
favorable, and accordingly, both species and individuals 
are numerous. As before mentioned, some of the spe- 
