PLEIOCENE FOSSILS. 45 



ARCA INCONGRUA— Say. 



Plate XVI., Figs. 5 and 6. 



A. testa sub-quadrato-orbiculari, incequivalva, radiatim costata ; latere buccali breviore ; 

 valvce sinistralis costis sub-crenatis, valvce dextralis costis omnibus crenatis. Area ligamenti 

 latiuscula ; umbonibus approximatis. 



Description. Shell somewhat squarely orbicular, inequivalve, radiately ribbed, ribs 

 about twenty-eight ; buccal side shorter, left valve with the ribs somewhat crenulated on 

 the buccal and anal margins ; ribs on the right valve all crenulated ; ligament area rather 

 wide ; umbones somewhat approximating. 



This, although one of the most common of our coast shells, is very rare in the fossil 

 state. 



Plate XVI., Fig. 5. Exterior of left valve, and hinge and teeth of right valve 



" 6. End view of fossil, showing outline of hinge and umbones. 



Locality. Darlington District. 



Museum, College of Charleston. 



