CRETACEOUS PERIOD. 171 



Ostrea prudentia Wbite. 

 Plate XIV, fig. 2 a, h, c, and (/. 



Shell neat and symmetrical for a species of this genus, suboval or sub- 

 circular in outline when adult, subcircular when young, moderately capa- 

 cious ; beaks small, usually distinct, and approaching so near to each other 

 when the valves are together as to leave only a narrow space between the 

 areas. Lower valve moderately deep ; area short and broad ; ligamental 

 groove short, broad, and distinct, bounded at each side by a rounded ridge ; 

 beak extending very slightly beyond that of the other valve ; scar of attach- 

 ment sometimes occupying' one-quarter of the outer surface, sometimes 

 extremely small, and sometimes a^^parently absent. Upper valve usually 

 flat or a little concave transversely, but a little convex longitudinally in adult 

 shells ; area a very little shorter than that of the other valve, moderately 

 convex or neaidy flat. 



Surface of both valves marked by distinct lines and laminae of growth, 

 but this species is rather less laminated and roughened than is usual in the 

 genus Ostrea. Somewhat numerous, corrugated, but rather indistinct radiat- 

 ing costse are usually to be seen on the ventral valve of young examples, 

 yet these corrugations seldom or never extend to the front half of old shells. 



Length, six centimeters ; breadth, five centimeters. 



This species is somewhat remarkable for its neatness of form and free- 

 dom from the crude extravagancies which species of this genus often exhibit. 



Position and locality. — Strata of the Cretaceous period ; east of Imprac- 

 ticable Ridge, Utah. 



Oentjs GRYPHEA. Lamarck, 1801. 



Gryphea Pitcheri var. Morton. 



Plate XVII, fig. 1 a, 6, c, d, e, and/. - 



Gryphea Pitcheri Morton, 1834, Syuop. Org. Remains, Cret. Group, 55. 

 Gryphea Pitcheri Rcemer, 1852, Kreidebikliing von Texas, 73, 

 Gryphea Pitcheri Conrad, 1857, U. S. «& Mex. Bound. Hurv., i, 155. 

 Gryphea dilatata Marcou, 1858, Geol. North America, pi. iv, f, 2. 



Shell reaching a moderately large size, very variable in shape, gen- 

 erally having an irregularly subovate marginal outline, often much longer 

 than broad, but sometimes shorter than broad. Larger valve capacious, 

 scaphoid, arcuate, more or less distinctly lobed, the posterior lobe occa- 

 sionally somewhat wing-like ; test rather thick ; umbo large, prominent, 



