8 
equally remarkable that the latter also contain the 
thinnest and most fragile as well as the most ponder- 
ous species. 
During a recent journey through Georgia and 
Alabama, I was enabled to learn the western limits 
of that class of shells of which Unto purpureus of 
Say may be considered the most characteristic. 
They occur in the Savannah, Oconee and Ocmulgee, 
but there they terminate, as Flint river, the first 
tributary of the Gulf of Mexico, which I crossed in 
my route, furnishes plicated shells and species iden- 
tical with some inhabiting the Ohio. I have not 
examined any of the rivers intermediate to the Flint 
and Ocmulgee, but as these two are only thirty miles 
distant from each other, and are not divided by a 
range of mountains, I think the fact that the Ocmul- 
gee contains the “eastern” and the Flint the “west- 
ern” species, substantiates the theory I have for 
years indulged, that the shells of the Atlantic streams 
differ in their general character from those of all the 
waters which flow into the Gulf of Mexico. I must 
observe, however, that a shell which I believe to be 
Unto purpureus , occurs in Flint river, in company 
with the ponderous and plicated shells, but west of 
this I never observed the species. A variety of U. 
deciivis , Say, was found in East Florida by Dr. 
Blanding, and U. parvus , Barnes, it is said, also 
