99 «(. RAFINESQUE’S 
[From the American Monthly Magazine and Critical Review, Vol. III, 
page 354, New York, 1818.] 
Discoveries in Natural History, made during a Journey through 
the Western Region of the United States, by Constantine 
Samuel Rafinesque, Esq. Addressed to Samuel L. Mitchiil, 
President, aud the other Members of the Lyceum of Natural 
History, in a letter dated at Louisville, Falls of Ohio, 20th 
July, 1818. 
rr Dickeabota ce or the Shells. I trust I have discovered likewise 
the greatest proportion of the shells of the Ohio, having already 
collected and described over 30 species, the whole of which appear 
to be new; they consist of 24 bivalve and 8 univalve shells. It is 
strikingly singular that those shells belong only to 3 genera, that the 
24 species of bivalve belong all to a single natural genus; and that 
those genera are all different from European fluviatile genera, which 
I have ascertained beyond a doubt. by the shells and animals thereof. 
I shall add the characters of these new genera. 
I. Poramitus.* Bivalve: Shell equivalve unequalateral, com- 
monly transverse, rugose transversely, sloping posteriorly, shape 
variable, margin thickened, two muscular impressions, an epidermis 
surrounding the margin by a membranaceous brim, connective 
oblong convex membranaceous. Ligament with two teeth on one 
side, and a deep furrow on the other, between two carina in the left 
shell, while the right shell has two unequal teeth, and two unequal 
carinas. 
Animal with a mantle open and bilobe, branchias as a second 
interior mantle, body compressed tough, two openings or siphons . 
anterior on each side, not tubular, one foot on each side commonly 
bilamellose, next to the openings. 
1. Sub-genus. Shell transverse, not truncated, thick and without 
knobs; 1. Potamilus latissimus; 2. P. violacinus ; S. P) nwers 
4. P. fasciolaris ; 5. P. phaiedrus; 6. P. ellipticus; 7. P. zona- 
lis; 8. P. obliquatus. 
2. Sub-genus. Shell transverse, truncated posteriorly, thick and 
without knobs. 9. Potam. retusus; 10. P. truncatus; 11. P. 
triqueter. 
3. Sub-genus. Shell transverse, thin, not truncated. 12. P. 
alatus ; 13. P. Jeptodon ; 14. P. fragilis ; 15. P. nervosus ; 16. P. 
fasciatus; 11. P. auratus. 
* If l remember right, this genus is also found in the Hudson river, where 3 or 4 species are to 
be seen, which have been mistaken for Mya or Cardium. 
