43^ 



NORTH AMERICAN INDEX FOSSILS. 



structure coarsely prismatic, with a thin, partial, nacreous lining; 

 byssiferous. Jurassic-Recent. 



199. P. petrina White. (Fig. 572.) Cretacic. 



Cardinal margin concave. Surface marked with strong growth 

 lines. 



Benton : Colorado, New Mexico? 



Fig. 571. Aviculopinna? peracuta, X \> Fig. 572. Pinna petrina, left valve, X \> 

 (Kansas Pal., VI.) (After Stanton, Bull. 106, U. S. G. S.) 



200. P. laqueata Conrad. (Fig. 573.) Cretacic. 



Nine to eleven strong, simple radiating ribs on the dorsal 

 portion of valves ; lower portion with very strong concentric 

 striae parallel to the margin. 



Ripleyan of New Jersey (Merchantville-Navesink), Gulf region. 



LXIII. Conocardium Bronn. 

 Very inequilateral. Anterior side short, truncated and flat- 

 tened laterally but produced along the hinge line into a beak-like 

 appendage ; this appendage, however, is usually broken away, 

 leaving a round hole in the flat anterior view. Posterior side 



