460 



NORTH AMERICAN INDEX FOSSILS. 



marked by 5 to 8 strong ribs, 3 of them much the strongest and 

 frequently outlining the shell as a more or less equilateral triangle. 

 Upper valve flat ; ribs nearly obsolete. Anterior and posterior 

 margins of both valves minutely crenulated. 



Texas (Trinity), northern Mexico (Washita). 

 261. 0. subovata Shumard. Comanchic. 



Large and massive, oblong-ovate. Both valves only slightly con- 

 vex. Beak of lower (left) valve elevated and projecting beyond that 

 of right. Surface coarsely imbricate and radially plicate, the ribs 



Fig. 610. Ostrea crenulimargo. a, external, and b, internal view of the 

 free valve. (After Roemer.) 



usually about 6 or 8, sometimes with tubercles or spines at top. 

 Average height 6 inches. 



Common in Fredericksburg and Washita of Oklahoma, Kan- 

 sas, Texas and New Mexico. 

 262. 0. quadriplicata Shumard. (Fig. 611.) Comanchic. 



Fig. 611. Ostrea quadriplicata. ( After Hill. ) 



Main radiating plications 4, produced at edge of valve. 

 Washita of Texas (Denison), Oklahoma and Kansas. 

 263. 0. soleniscus Meek. (Fig. 612.) Cretacic. 



Long and narrow. Often very large and thick. 



