Geographical Distribution of Certain Fresh-Water Mollusks. 165 
ties equal to that of the Unionide, The species of Goniobasis (?) 
Viviparus, Physa, and Planorbis, are all cases in 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 propa- 
toris: “It is 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. Let this be as it may, I 
am convinced that the origin of these Tertiary and Cretaceous forms, 
is to be sought in a Paleozoic progenitor, whose probable starting 
point was in regions adjacent to the western Archean. While the 
species of fresh-water habitat may have persisted since the Carbon- 
iferous, in all the region between the Appalachians and the Mississippi, 
much of that portion of geological time has been fatal to such ex- 
istence in the region west of the same stream; and though Mr. Tryon 
speaks of the Mississippi as a barrier to the westward distribution 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 muddy waters 
of the Mississippi are an effectual barrier, in a general way, accidental 
transportation or a few cases of actual traverse, that we can not doubt 
must have taken place, would have furnished abundant materials for 
spreading the species through our western rivers, ifthe conditions had 
been favorable; but they were not favorable, and consequently no such 
distribution has taken place. Hence it is, that the few species of shells 
inhabiting those streams, seem to me more likely to be the descend- 
ants of ancestry of an old date, and their general correspondence in 
form to the Ohio type, points to their community of origin. The 
fauna E is here wanting; nor has it any representative. When we 
come to the consideration of the down stream distribution of the 
species east of the Mississippi, we find the Strepomatide, as 
represented by their most characteristic genera, and Fauna E 
of the Unionide, to have a barrier in that direction. Here they 
cease, and beyond it, in the Tennessee, Cumberland, etc., we find 
mainly the Fauna D. Since this fact is general, it becomes one of 
high significance in this discussion, and stands as a unique evi- 
dence in favor of some of the suggestions here made; and it shows, 
conclusively, that continuous water is not the only condition of 
molluscan distribution; and that the present station of Zo, Goniobasis, 
Anculosa, etc., in mountain streams, and in the more rapid portions of 
