ARCADiE. 19 



Genus, ARCA. — Linn. 



ARC A INCONGRUiV. 

 Plate IV. Figs. 1 and la. 



Area incongrua, Saij, Jour. Acad. Nat. Sci., Vol. 2, p. 268. 



Area incongrua, De Kay, Zool. New- York, Art. Mollusca, p. 178. 



Area incongrua, Ravenel, Cat. Coll. Shells, p. 5. i 



Area incongrua, L. R. Gibbes, Tuomey's Geol. So. Ca., appendix, p. xxi. 



Area incongrua, Tuomey Sf Holmes, Pleiocene Foss. So. Ca., p. 45, pi. 16, figs. 5 and 6. 



Area incongrua. Say's Conch. U. S., (Binney,) p. 93. 



Description. Shell squarely orbicular, inequivalve, radiately ribbed ; ribs twenty-six 

 to twenty-eight, buccal side shortest ; left valve with crenulated ribs on the buccal and 

 anal margins ; right valve has all the ribs crenulated; ligament area wide, lanceolate; 

 umbones approximating. 



This is the most common shell on our sea coast, and very numerous in the Post-Pleiocene 

 beds. It is rare in the Pleiocene. 



Plate IV. Fig. 1, Valves, natural size. 



" 1«, View of hinge a7id beaks. 



Locality. Charleston; Simmons'. 



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



ARCA AMERICANA. 



Plate IV. Figs. 2 and 2a. 

 J 



Area amerieana. Gray, Reeve, Conch. Icon., pi. 4, fig. 21. 



Area pexata, Ravenel, Cat. Coll. Shells, p. 5. 



Area pexata, L. R. Gibbes, Tuomey's Geol. So. Ca., appendix, p. xxi. 



Description. Shell thick, oblong-ovate ; buccal margin shortest, and regularly rounded ; 

 anal margin obliquely truncated ; beaks not ventricose, but prominent, nearly touching, 

 space between them very narrow ; ligamental area narroV, and terminating anteriously at 



