22 rafinesque's 



[From the American Monthly Magazhie and Critical Review, Vol. Ill, 

 page 354, New York, 1818.] 



Discoveries in Natural History, made during a Journey through 

 the Western Region of the United States, hy Constantine 

 Samuel Rafinesque, Esq. Addressed to Samuel L. Mitchill, 

 President, aud the other Members of the Lyceum of Natural 

 History, in a letter dated at Louisville, Falls of Ohio, 20th 

 July, 1818. 

 [355] 

 4. Conchology 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 aud animals thereof. 

 I shall add the characters of these new genera. 



I. PoTAMiLUS.* 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 ; 3. P. niger ; 

 4. P. fasciolaris ; 5. P.phaiedrus; 6. P. elliptic us; 1. P. zoncc- 

 lis ; 8. P. obliquatas. 



2. Sub- genus. Shell transverse, truncated posteriorly, thick and 

 without knobs. 9. Potam. retusus ; 10. P. truncatas ; 11. P. 

 triqueter. 



3. Sub-genus. Shell transverse, thin, not truncated. 12. P. 

 alatus ; 13. P. leptodon ; 14. P. frag His ; 15. P. nervosus ; 16. P. 

 fascial us ; 17. P. auratus. 



* If I remember right, this genus is also found in the Hudson river, where 3 or 4 species are to 

 be Been, which have been mistaken for Mi/a or Cardium. 



