und the probable causes of their Variations. 211 
mergence than took place in this region during the Tertiary, 
would exterminate many contemporary species in the lower 
part of its drainage. In such a case, this isolated remnant, 
ique and strange, would present us with a problem for con- 
sideration 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 drainage 
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 compara- 
tively little change; or, possibly, 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 indicate and in many cases it is not diffi- 
cult 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 varieties equal to that of the Unionide. e species of 
Goniobasis (?), Viviparus, Physa and Planorbis, are all cases 
im point; but one can not help seeing how closely the three 
genera last mentioned are related in all these fossil forms to 
Species now living; and it seems that Dr. White’s remark, 
accompanying the description of the Anodonta propatoris, “It 
18 not to be denied that its separate specific identity is assumed 
from its known antiquity, rather than proved by its structure 
and form,” might have been, with still greater significance, 
written of many of these fossil Viviparide and Limneide. t 
this be as it may, I am convinced that the origin of these Ter- 
lary and Cretaceous forms is to be sought in a Paleozoic pro- 
genitor, whose probable starting point was in regions adjacent 
to the western Archean. While the species of fresh-water 
habitat may have persisted since the Carboniferous, in all the 
region between the Appalachians and the Mississippi, much of 
that portion of geological time has been fatal to such existence 
in the region west of the same stream; and thou r. Tryon 
Speaks of the Mississippi as a barrier to the westward distribu- 
tion of species, it seems to me that the cause is really to be found 
in the character of the western tributaries as well ; for while the 
