Geographical Distribution of Certain Fresh-Water Moillusks. 161 
number of described ‘‘species” of the genus Unio; for among the forms 
filched from these prolific streams, malacological enthusiasts have dlis- 
ported themselves as species-makers, until the crying need of our times 
is an honest, impartial, and thorough review of the whole subject. 
The approximate boundaries of Fauna E may be placed between the 
Ohio river on the north, the Tennessee on the south, the Appalachians 
and the Mississippi. One fact is of curious import here; and it de-. 
serves to be put upon record in this discussion, and in this place. In 
his last edition of his Synopsis of the Family Unionide, 1870, which 
he tells us is his “most important work,” Mr. Lea makes the follow- 
ing remarkable statement, the truth of which he had abundant oppor- 
tunities to verify; “although I have examined critically, and pub- 
lished descriptions of the soft parts of 254 species of this family, and 
have often dissected 50 to 100 of the same species, I can not see, as 
yet, any useful division that could satisfy the student or the adept, 
which can be made by systematic difference in the organic forms of the 
soft parts.”” This means, I suppose, that the differences of the soft 
parts are so small as to afford no safe basis upon which to predicate 
classification. I may add to this, that the most intimate study of the 
anatomy of different species of the Limneide and Strepomatide, has 
convinced me beyond resonable doubt, that specific differences, sup- 
posed to be indicated in the shells, do not extend to the animals them- 
selves, so far as these studies go to show. I have now in course of 
preparation a memoir on this subject, which I hope soon to publish 
with accurate anatomical illustrations. Here is one of those strange 
‘facts, standing at the very threshold of the question of evolution, which 
finds a parallel in the Lingula and the Rhizopod. 
We may now venture upon a few suggestions, to which these facts 
give rise. Clearly the oldest shell fauna upon the continent would 
have naturally inhabited Archzean regions; and as it is altogether 
likely, from chemical facts associated with the deposit of iron ores, and 
the presence of graphite in the older rocks of the continent, as pointed 
out by Prof. Dana and Dr. Hunt, that organic life may have existed 
to an extent not yet determined by fossils actually discovered as such, 
I think we do not pass beyond the bounds of probability in assigning 
to Fauna A a very remote antiquity. From its original locus, it has 
spread to the limit of suitable conditions, a limit undergoing constant 
variations, perhaps, through the geological ages, but which has been 
determined by boundaries mainly fixed by true geological causes. 
Through adaptation this fauna has, in a few cases, overstepped its 
