20 POST-PLEIOCENE FOSSILS. 



the beaks; ribs thirty-seven to thirty-nine, radiating, flat, and divided in the middle by a 

 longitudinal line, which becomes obsolete on the umbones ; margins strongly impressed 

 within. 



This is a common species on the coast of North and South-Carolina, and Georgia ; it 

 has been long confounded with Arca pexata, (Say,) a shell limited in its Southern range 

 to Cape Hatteras ; A. Americana, is readily distinguished from it by the truncated anal 

 margin ; the greater number of ribs, thirty-seven to thirty -nine, and less prominent 

 umbones ; the spaces between the ribs are deeper and narrower. 



Like A. pexata, (Say,) and A. holmesii, (Stimpson,) it has the hinge-line terminating 

 at the beaks. 



Numerous and fine specimens are obtained from all the Post-Pleiocene beds of the 

 State. 



Plate IV. Fig. 2, Exterior, and hinge. 

 " 2a, End view. 



Locality. Charleston; Simmons'. 



Museum, College of Charleston; Cabinet F. S. H. 



ARCA LIENOSA. 

 Plate IV. Figs. 3 and 3a. 



Area lienosa, Say, Am. Conch., pi. 36. 



Arca lienosa, Tuomey 6? Holmes, Pleiocene Foss. So. Ca., p. 40, pi. 15, fig. 2. 



Arca lienosa, Say's Conch. U. S., (Binney,) p. 192. 



Description. Shell equilateral, transversely oblong, inflated, ribbed, rugose; buccal 

 side produced, angular; anal side elongated, obliquely truncated; ribs about thirty -five, 

 unequal, channelled ; ligament area, wide, sulcate ; sulci slightly diverging, but somewhat 

 parallel to the hinge ; umbones distant. 



We are not aware of this species now existing, except upon the coast of Florida ; it is 

 rare in the Post-Pleiocene, but more abundant in the Pleiocene of South-Carolina. 



Plate IV. Fig. 3, Shell, natural size. 

 " 3a, View of hinge. 



Locality. Simmons'. Museum, College of Charleston; Cabinet F. S. H. 



