STANTON.] OSTREID.E. 55 



of the species in that list have been found to occur in the Colorado for- 

 mation, and I have included all of them in its fauna, though some oi 

 them may have been found at a lower horizon than is usually regarded 

 as the base of that formation. 



Ostrea anomioides Meek. 



PI. I, Figs. 5 and 6. 



Ostrea 'anomioides Meek, 1873, Ann. Kept. U. S. Geol. Sur. Terr, for 1872, p. 488; 

 White, 1880, idem for 1878, p. 10, PI. 11, Figs. 4a, b; 1884, 4th Ann. Kept. U. S. 

 Geol. Sur., p. 291, PI. 39, Figs. 4 and 5. 



Original description: 



" Shell rather small, very thin, depressed plano-convex, and without 

 any visible scar of attachment, varying from ovate to circular ; rounded 

 or sometimes a little straightened on the hinge margin; beaks scarcely 

 projecting beyond the outline of the cardinal margin. Lower valve 

 very shallow; cartilage pit unusually small, shallow, and short. Up- 

 per valve almost perfectly flat; cartilage attachment even shorter than 

 that of the other valve, and slightly convex on its inner margin. Mus- 

 cular scars unknown; surface of both valves with small regular con- 

 centric wrinkles most distinctly marked on the central region. 



" Greatest diameter of one of the largest oval specimens, 1.70 inches; 

 breadth, 1.40 inches; convexity, 0.23 inch. 



" This species is remarkable for the thinness of the shell, the slight 

 concavity of the under valve, and the flatness of the upper, as well as 

 for its rounded or slightly straightened cardinal margin, and the absence 

 of any scar of attachment, or of any traces of muscular impressions 

 within. These external characters, and the regular small concentric 

 wrinkles, give the exterior of the lower valve of circular specimens 

 somewhat the appearance of a Lucina or Dosinia; while in other in- 

 dividuals it looks more like an Anomia or Placuna. 



" Locality and position. — Missouri river, below Gallatin city, Montana. 

 Cretaceous." 



Ostrea congesta Conrad. 



PI. II, Figs. 2, 3, and 4. 



Ostrea congesta Conrad, 1843, Nicollet's Kept, of Explorations in the Northwest, p. 

 167; Hall, 1856, Pacific R. R. Reports, vol. in, p. 100, PI. 1, Fig. 11; Meek, 1876, 

 U. S. Geol. Sur., vol. ix, p. 13, PI. 9, Figs, la-/; White, 1884, 4th Ann. Rept. U. 

 S. Geol. Sur., p. 294, PL 39, Figs. 11, 12, 13. 



Prof. Meek's description is as follows : 



" i Shell elongated; upper valve flat; lower valve ventricose, irregu- 

 lar; umbo truncated by a mark of adhesion. 7 (Conrad.) 



"This is a small, thin shell, the individuals of which are often 

 crowded together in considerable numbers, so as to assume quite irreg- 

 ular forms. In cases where the individuals had room to grow without 



