OF THE UNITED STATES. 59 
Very distinct from any of the preceding species. Found 
by Mr. Conrad in the calcareous strata, near the Eutaw 
Springs, South Carolina. 
7. P. membranosus, (S.G. M.) PI. x, fig. 4. 
Specific character. Orbicular, convex, somewhat gibbous in 
the centre, with about eighty distinct coste, the alternate ones 
being smaller. 
Found by Mr. Conrad in the calcareous strata of South 
Carolina, where it is abundant. 
8. P. Poulsoni, (S.G.M.) PI. xix, fig. 2. 
Specific character. Suborbicular; superior valve flat; ribs 
fourteen, not profoundly elevated, with crowded wrinkled strie; 
inferior valve ventricose, with prominent rounded ribs, the in- 
tervals striated; ears subequal. 
Common in the newer cretaceous deposits near Clai- 
borne, Alabama. 
I have much pleasure in naming this shell after my 
friend Charles A. Poulson, Esq., of this city. 
 PLAGIOSTOMA. 
1. P. dumosum, (S.G.M.)_ PI. xvi, fig. 8. 
Specific character. Shell obovate, slightly ventricose, with 
nine or ten slightly elevated ribs, armed with long spines on 
each valve; spines flattened, and with a longitudinal groove be- 
neath; intervals beneath the ribs, with two or three slightly 
prominent longitudinal lines. 
