164 Cincinnati Society of Natural History. 
of the south and southwest, we shall find that when we have removed 
the Ohio types from the lists, very few valid species remain. How ab- 
solutely true this is, and how great the synonymy of these shells, fam 
sure is not the well understood fact in American malacology that it 
ought to be. There can be little doubt that the distinctively Ohio 
types, these widely distributed, and so greatly differentiated, antedated 
any other forms occupying the same region with them. * But other 
groups, during the mutations of the geological ages, left their remnants 
which have spread over the same area. The persistent species have_ 
either less tendency to variation, or the precise circumstances to call 
out such latent energies have not yet been brought into active account; 
while other forms, for opposite reasons, present us an infinity of varie- 
ties, always easily recognized, and of the derivative character of which 
no person who has investigated this subject can have any doubt. 
In this connection the isolated fauna of the Coosa, to which reference 
was made in the previous article must not be neglected. This stream 
flows through a comparatively limited drainage. It contains two 
genera, Schizostoma and Tulotoma, represented by thirty species, 
that have not yet been found outside of it; and this in a region where 
every stream contains an abundance of Strepomatide, How easy for 
a slight geological disturbance to obliterate the record of their exis- 
tence; how easy to have an isolated remnant of this unique fauna left 
in the upper reaches of this mountain stream, when a less submergence, 
than took place in this region during the Tertiary, would exterminate 
many contemporary species in the lower part of its drainage. In sucha 
case, this isolated remnant, unique and strange, would present us 
with a problem for consideration like that of the Unio spinosus. This 
single example well represents the principle to which this article 
points, and shows how readily, in earlier times, when systems of drain- 
age were comparatively limited, and opportunities for the spread of 
species were correspondingly less, there might have been many cases like 
that of the Coosa, during the various Epochs, which left remnants of 
their shell-fauna; and those remnants, which had less tendency to 
variability, have persisted with comparatively little change; or, possi- 
bly, the changes have been in a direction which did not characterize 
other groups with which they were associated, leaving them distinct. At 
all events, the faunas are plainly indicated, and in many cases it is not 
difficult to point out central forms, around which they seem to be 
clustered. The various other genera of Fresh-Water Shells, found in 
the western deposits above mentioned, all exhibit a tendency to varie- 
