4o8 



NORTH AMERICAN INDEX FOSSILS. 



Fig. 526. Trigonarca obliqua, 0, left valve of a large specimen ; b, internal mold 

 of left valve ; d, posterior view of same, shell on right valve ; r, dorsal view of a smaller 

 specimen ; <?, hinge of right valve. (After Stanton.) 



area with minute vertical striae. 

 Cretacic. 



Denticles very fine and crowded. 



118. B.? siouxensis (Hall and Meek). Cretacic. 



Anterior border vertically subtruncate but convex along the 



middle and rounding rather abruptly into the base and hinge line. 



Posterior side abruptly truncate above, narrowly rounded below. 



Beaks subcentral, nearer 

 the posterior end, slightly 

 or not at all oblique. 

 Distinguished from other 

 species by its large size 

 (length nearly one inch 

 and height slightly less) 

 and by its straight basal 



Fig. 5 2 7> Breviarca saffordi, enlargement of margin, 

 hinge and exterior natural size. (After Whitfield, Dakotan . South Da- 



Pal. N. J., I.) 



kota, Texas (Woodbine). 



^k 



119. B. saffordi (Gabb). (Fig. 527.) 

 Surface covered with fine radiating striae. 

 Ripleyan of New Jersey (Woodbury), Tennessee. 



Cretacic. 



