NORTH-CAKOLINA GEOLOGICAL SUEVEY. 



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 

 line, 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 h'gure 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. TKANSVERSA. 



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 



