140 rAL/EONTOLOGY. 



C11P]TACE0US FOSSILS. 



MOLLUSC A. 



OSTREID^.. 



Genus OSTREA, Linn. 

 OsTREA (undt. sp.). 



Plate U), tigs. 10, .and 10 a, 6, c. 



Shell of about medium size and thickness, more or less elongate-sub- 

 ovate, tapering to the beak, which is usually abruptly pointed, and often 

 bent a little to the left or to the right, generally compressed and subequi- 

 valve. Lower valve rather shallow ; ligament-area triangular, with its mesial 

 furrow usually deep; surface merely showing apj^ressed imbricating lamina? 

 of growth, without any traces of radiating ridges, plications, or striai. Upper 

 valve a little more flattened, or sometimes nearly as convex as the other, 

 but rather less concave within ; beak usually more obtuse, and the ligament- 

 area often proiiortionally a little shorter, with its mesial ridge well defined; 

 lateral in;u-gins often thickened and crenated near the beaks; surface much 

 as in the other valve. 



Length of a medium-sized specimen, about 2.80 inches: breadth, L90 

 inches; convexity of the two valves, about 1 inch. 



In first preparing this report, 1 merely gave figures of this Oyster with- 

 out a specific name. Subsequently, in revising portions of the report, with- 

 out having the type-specimens at hand for comparison, I was impressed with 

 the similarity of this shell, as figured on our plate, to a species that I had 

 in the mean time described in one of Dr. Hayden's reports from Wyoming, 

 under the name 0. Wyomingensis, and placed that name with a mark of 

 doubt opposite its number on the explanations of the plate, while I also in 

 the same way mentioned it in a list of Coalville species. Having since 

 made a direct comparison of the specimens from the two localities, I am led to 

 doubt their specific identity, though they are certainly very much alike. The 

 Coalville specimens have the beak of tlu; under valve less curved upward, 

 and the lateral margins of the only upper valve I have seen from that 



