54 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull .106. 



MOLLUSCA. 

 PELECYPODA. 



OSTREIDAE. 

 Genus OSTBEA Linnaeus. 



OSTREA PRUDENTIA White. 



PI. I, Figs. 3 and 4. 



Ostrea prudentia White, 1876, U. S. Geol. Sur. West 100th Meridian, vol. iv., p. 171, 

 pi. 14, Figs. 2a-d; 1884, 4th Ann. Rept. U. S. Geol. Sur. p. 299, PL 40, Figs. 5 

 and 6. 



Compare Ostrea patina Meek aud Hay den, 1856, Proc. Acad. Nat. Sci. Phila., p. 277; 

 Meek, 1876, U. S. Geol. Sur. Terr., vol. ix, p. 16, plates 10 and 11. 



Original description : 



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

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

 capacious; 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 attachment sometimes occupying one- 

 quarter of the outer surface, sometimes extremely small, and some- 

 times apparently 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 

 nearly flat. 



" Surface of both valves marked by distinct lines and laminsB oi 

 growth, but this species is rather less laminated and roughened than 

 is usual in the genus Ostrea. Somewhat numerous, corrugated, but 

 rather indistinct, radiating costae 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, 6 cn \; breadth, 5 cm . 



"This species is somewhat remarkable for its neatness of form and 

 freedom from the crude extravagances which species of this genus often 

 exhibit. 



"Position and locality. — Strata of the Cretaceous period, east of Im- 

 practicable ridge, Utah." 



This species so closely resembles some specimens of Ostrea patina 

 M. & H., from the Montana formation, that 1 have beeu inclined to treat 

 it as a synonym of that species. In' the geology of the Uinta mountains 

 Dr. White gives it in the list of species belonging to the Henry's Fork 

 group, which has since been correlated with the Dakota, but a number 



