848 THE AMERICAN NATURALIST. [Vor. XXXV. 
confined, however, to those portions of the Coosa and branches 
that approach to east Tennessee."  Lithasia and Pleurocera 
are other genera divided in their distribution between the two 
systems. In the genus Strephobasis one species (Bitzniata) 
is found in the Black Warrior of Alabama drainage and in the 
Tennessee River at Chattanooga. 
In the Goniobasic section there are two types of shells: one 
group is characteristic of the Tennessee and the other of the 
Coosa. In the genus Goniobasis, about ten of the elevated 
smooth species are found in the Tennessee, and about the same 
number in the Alabama system. Tryon (/oc. cit., p. xlviii) says 
of this genus : * There are over sixty species in the group which 
I have designated as ‘compact and ponderous’ for want of a 
better name. They are essentially a distinct group from the 
other Goniobases, and all the species except three are peculiar 
to the branches of the Alabama River." | 
Consequently, we see that this family reaches its maximum 
development in two distinct river systems, the upper Tennessee 
and the Coosa-Alabama, which formerly formed one river sys- 
tem (Fig. 5), and that by the diversion westward of the upper 
Tennessee, by the migration of its divide, this fauna has been 
split into two parts, each part having several genera, as well as 
species, in common. This process has led to an isolation of 
the Coosa-Alabama fauna and a mixing of the waters of the 
upper Tennessee with those of the Mississippi valley. In 
faunal studies some of the most striking peculiarities of the 
Tennessee River are very apt to be overlooked if we consider 
it a part of the Mississippi system only. - % 
That these two systems have been separated for a consider- 
able period of time, or that these forms have evolved very 
rapidly, is shown by the fact that it is the genera or groups of 
species which are found common to both streams rather than 
the individual species. 
The family Pleuroceridz is not the only group which has 
divided by the diversion of the waters of the Tennessee. The 
family Viviparidae has a geographical distribution very similar 
to.that of the Pleuroceridz, its greatest variety being found In 
southeastern United States. The genus Tulotoma is confin 
