SPECIES OF THE FRESH- AND BRACKISH-WATER LIGNITE BEDS.* 



MOLLUSCA. 



LAIELLIBRANCJIIATA. 



MONOMYARIA. 



OSTREIDiE. 



Genus OSTEEA, Linnaeus (see page 10). 



Ostrea glabra, M. & H. 



Plate 40, figs. 2, a, b, c, d. 

 Ostrea glabra, Meek ami Haydeu (1857), Proceed. Acad. Nat. Sci. Pbilad., IX, 146. 



Shell usually subovate, slightly arcuate laterally, narrowing toward the 

 beaks, and rounded at the other extremity ; generally moderately thin ; sur- 

 face nearly smooth, or only marked by small, very obscure, irregular, concen- 

 tric wrinkles of growth. Lower valve deep, and provided with a small, 

 rather short, ligament-area; beak angular, sometimes pointed, often a little 

 deformed by the scar of attachment, generally abruptly curved to one side; 

 muscular impression obliquely oval. Upper valve flat or a little concave, and 

 truncated at the extremity of the beak. 



Length, 3 inches; breadth, about 1.50 inches; concavity of lower valve, 

 0.75 inch. 



This species presents very few distinguishing features. It is rather 

 thin and subovate in general form, with its under valve comparatively deep, 

 and the upper flat, or a little concave. The beak and area of the former are 



* Owing to the conflicting nature of the evidence in regard to the age. of these deposits, I have 

 concluded to describe here together, iu one separate section, all of the known invertebrate remains from 

 the whole of the Fresh- and Brackish-water Lignite beds of the Upper Missouri. Having already, in 

 the general introductory remarks, discussed the question respecting the probable relations of these strata 

 to the Cretaceous and Tertiary epochs, it is unnecessary to add anything further on that subject here. 



509 



