192 SYSTEMATIC PALEOJs^TOLOGT 



Shell small, roughly triangular; beak high, in some cases slightly 

 tAvistecl; raised lamellae absent or confined to the extreme periphery; 

 radial grooves, becoming stronger toward the periphery. 



Length, 85 mm.; width, 65 mm. 



Occurrence. — Aquia Formatio]s^. Fort Washington, Aquia Creek. 



Collections. — Maryland Geological Survey, L'. S. Xational Museum, 

 Philadelphia Academy of Natural Sciences. 



OsTEEA SELLAEFOEMis Conrad. 

 Plates XLVIII, XLIX. 



Ostrea sellaeformis Conrad, 1832, Fossil Shells of the Tertiary, No. 2, p. 27, pi. xiii, 



tig. 2. 

 Ostrea sellaeformis Morton, 1834, Sj-n. Org. Rem. Cretaceous Group, App., p. 6. 

 Ostrea sellaeformis Conrad, 1842, Proc. Nat. Inst., Bull, ii, pp. 192, 193, pi. i, tig. 1. 

 Ostrea sellaeformis H. C. Lea, 1848, Proc. Acad. Nat. Sci. Phila., vol. iv, p. 103. 

 Ostrea sellaeformis Conrad, 1865, Amer. Jour. Conch., toI. i, p. 15. 

 Ostrea sellaeformis Conrad, Smith. Misc. Coll. (200), p. 3. 

 Ostrea sellaeformis Heilprin, 1883, White's Fossil Ostreidae ; Fourth Ann. Kept. 



U. S. Geol. Survey, p. 311, pi. Ixii, figs. 1, 3; pi. Ixiii, fig. 1. 

 Ostrea sellaeformis Clark, 1895, Johns Hopkins Univ. Circ, vol. xv, p. 5. 

 Ostrea sellaeformis Clark, 1896, Bull. 141, U. S. Geol. Survey, p. 87, pi. xxxv, figs. 



la-ltZ; pi. xxxvi, figs, la, 1&. 



Description. — " Oblong, convex, thick and ponderous, lobed; one side 

 of the larger valve profoundly sinuous and the opposite side gibbous; 

 smaller valve sinuous and little convex; dorsal margin long and slightly 

 arched, with both extremities obtusely rounded." Conrad, 1833. 



This form although very abundantly and characteristically developed 

 in the valleys of the James and Pamunkey rivers, occurs generally in 

 the Potomac Valley and farther north only as small specimens, almost 

 indistinguishable from the young of 0. compressirostra. 



Height, 60 mm.; width, 10 mm. (largest Maryland specimen). 



Occurrence. — Xanjemoy Fokmatiox. Popes Creek, 1^ miles above 

 Popes Creek, 2^ miles above Popes Creek, East of Port Tobacco, Upper 

 Marlboro (deep cut near Chesapeake Beach E. E. station), AYoodstock. 



Collections. — Johns Hoj^kins TJniversitj', Philadelphia Academy of 

 Natural Sciences, Maryland Geological Survey. 



