INVERTEBRATE PALAEONTOLOGY. 13 



and a flat upper one. Impressions of the surface of the latter show merely 

 small imbricating marks of growth ; while internal casts of the former retain 

 a few obscure concentric undulations, crossed by a few oblique, faintly-marked, 

 radiating ridges. 



Length of largest internal cast seen, 1.08 inches; breadth, 0.65 inch; 

 convexity, about 0.30 inch. 



Locality and position. — From a rough ferriginous sandstone, belonging 

 to the Dakota group, at a locality twelve miles southwest of Saliha, Saline 

 County, Kansas. Collected by Prof. B. F. Mudge. 



Ostrea congcsta, Conrad. 



Plate 9, rigs. 1, a, b, c, d, e,f. 



Ostrea congesta, Conrad (1843), Nicollet's Report of Explorations in the Northwest, 167. — Hall ami Meek 

 (1854), Mem. Am. Acad. Arts and Sci. Boston, VIII,(n.s.),405. — Meek and Haydeu (Nov., 

 1856), Proceed. Acad. Nat. Sci. Philad., 286.— Hall (1856), Pacific R. R. Reports, III, 100, 

 pi. 1, iig. 11. 



"Shell elongated; upper valve flat; lower valve ventricose, irregular; 

 umbo truncated by a mark of adhesion.'' (Conrad.) 



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

 together in considerable numbers, so as to assume quite irregular forms. In 

 cases where the individuals had room to grow without interruption, the young 

 shell is usually found to be of an ovate form, and attached by the whole under 

 surface of the lower valve, the beak of which is pointed, provided with a small 

 triangular area, and usually turned a little to the left. In this form they con- 

 tinued to grow, to lengths varying from 0.25 to 1 inch, when the margins 

 were abruptly deflected upward at right angles to the flat, attached base, and 

 produced in this direction often for as much as an inch or more; the greatest 

 extension being on the lateral margins and at the extremity opposite the 

 beaks. When seen at this stage of their growth, separated from the body to 

 which they were originally attached, and lying partly embedded in the matrix, 

 with the beak side down, they look like short cylindrical tubes, with one end 

 abruptly truncated and closed by the flat surface of attachment; so that what 

 was originally the whole under surface of the valve now appears like the 

 truncated umbo. 



The other valve is quite flat, or sometimes a little concave, and always 

 retains the form possessed by the attached valve at the time its margins 

 became deflected upward, after which it seems to have increased very little 

 in size. Its umbo is usually a little less pointed than that of 'the other valve, 



