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 costae, the alternate ones 

 being smaller. 



Found by Mr. Conrad in the calcareous strata of South 

 Carolina, where it is abundant. 



8. P. Pouhnni, (S. G. M.) PI. xix, fig. 2. 



Specific character. Suborbicular ; superior valve flat; ribs 

 fourteen, not profoundly elevated, with crowded wrinkled striae; 

 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.) PL 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. 



