NORTH-CAROLINA GEOLOGICAL SURVEY. 285 



duced ; umbones incurved, distant ; ligament area crossed by 

 transverse lines. 



This shell has about thirty-one principal ribs, with inter 

 vening raised lines, and transversely marked by lines of growth. 



A. CENTENARIA. (Fig. 205.) 



Shell sub-quadrate and ovate, nearly straight and slightly 



FIG. 205. 



contracted at base ; ribs tine, alternating in size ; margins 

 rounded ; beaks approximate ; hinge area narrow ; margins 

 entire. 



The- striae or ribs in this species are very numerous and 

 tine, while these together with its quadrangular form will 

 serve to distinguish it from others of the same genus. Com 

 mon in the miocene of North-Carolina. The tigure was 

 drawn from a specimen obtained from the indurated sand be 

 neath the miocene bed at Elizabethtown, Bladen county, and 

 is referred to the centenaria but with doubt. 



A. IDONEA. 



Subcordate inequivalve ventricose ; elongated and only 

 slightly oblique; beaks very prominent and distant; ribs 

 about twenty -five, crenulated, or transversely ridged; hinge 

 area wide and marked by divergent striae or channels. Com 

 mon in the miocene of North-Carolina. 



A. TRANSVERSA. 



Shell rather thin, subrhomboidal, rounded with about 

 thirty-two ribs ; area rather narrow, with two or three undu 

 lated grooves. Common in the miocene, and still living upon 



