516 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



! n i o p r i s c u s , M & H 



Plate 43. figs. 8, a,b,c,d. 

 Uniopriacus, Meek and Hayden (1856), Proceed. Acad. Nat. Sci. Pbilad., VIII, 117. 



Shell transversely-subovate, thin, apparently moderately compressed ; 

 anterior side regularly rounded ; posterior side long, and abruptly rounded 

 or subangular below ; cardinal outline arcuate, forming a broad descending 

 curve behind the beaks to the posterior basal extremity ; basal margin straight 

 behind, convex in advance of the middle, and rounding up regularly in front; 

 beaks very small, rising little above the cardinal border, located nearer the 

 anterior end than the middle, and ornamented by small, very regular, con- 

 centric wrinkles ; other portions of the surface only marked by very obscure, 

 irregular lines of growth. 



Length, 2.99 inches; height, 1.57 inches; convexity unknown. 



This shell has much the aspect of an Anodonta externally; but several 

 fragments found associated with it, presenting the same external characters, 

 and apparently belonging to the same species, show enough of the hinge to 

 prove it to be provided with teeth as in the genus Unio. Some of these 

 fragments show the concentric wrinkling of the beaks very distinctly, and 

 also have one or two small, raised, radiating lines, which extend from the 

 back part of the beaks obliquely backward and downward across the posterb- 

 dorsal region of the immediate umbones. These markings, however, are only 

 seen on the beaks, excepting in young shells. The only entire adult speci- 

 men yet seen is unfortunately flattened by pressure, so as to leave some 

 doubts in regard to the extent of its original convexity ; though it appears to 

 have been but moderately gibbous (see fig. 8, d, pi. 43). 



In form and general appearance, it is somewhat similar to U. Duboquii 

 of Coquand, from the Upper Eocene deposits of the Province of Constantine 

 (Mem. Geol. Soc. of France, V (sec. ser.), part 1, pi. v, tigs. 5 and 6), but differs 

 in being higher in proportion to its length, and entirely destitute of concen- 

 tric undulations excepting on the immediate beaks. It is more nearly related 

 to a species described by me in I860 from the brackish-water beds at Bear 

 River, Wyoming, under the name U. vetustus. Indeed, these forms are so 

 very similar that I was led at one time to think that they might be identical. 



Locality and position. — On Yellowstone River, forty miles above its 

 mouth ; from the Fresh- and Brackish-water Lignite series, apparently at the 



