GEOLOGICAL SURVEY OF THE TERRITORIES. 303 



Locality and position: Near Medicine Bow Station, Union Pacific 

 Railroad; Fort Pierre Group of the Upper Missiouri cretaceous section. 



UNIO (BAPHIA?) NEBRASCENSIS. (MEEK.) 



Shell attaining a medium or larger size, cuneate-subovate, being in 

 the adult very gibbous anteriorly and cuneate behind ; anterior side 

 very short and rounded ; posterior sloping above obliquely from the end 

 of the hinge to the posterior basal extremity, which is narrowly rounded; 

 basal border sinuous behind the middle, and convex in front of it ; car- 

 dinal margin rather short and nearly straight or slightly arched ; urn- 

 bones very gibbous, but depressed, oblique, incurved, and placed near 

 the anterior extremity ; posterior umbonal slopes subangular from the 

 beaks obliquely backward and downward to near the middle, beyond 

 which they are continued as broadly-rounded ridges to the posterior basal 

 extremity ; below and parallel to these ridges there are also, on the flanks, 

 one or two large, oblique, irregular, rounded plications or undulations, 

 that continue on to the sinuous posterior basal margin, to which they 

 sometimes impart a distorted or waved appearance. Surface otherwise 

 smooth, excepting moderately-distinct lines of growth, which are strongly 

 undulated in places, as they cross the oblique plications of the flanks. 



Length, 4.10 inches; height, 2.36 inches; convexity, 2.07 inches. 



Specimens of this large shell were brought by Dr. Hayden from Ne- 

 braska, some years back, but I have delayed describing it, with the hope 

 that other specimens would be found that might show the nature of the 

 hinge. It has so much the external aspect of the Unionidw as strongly 

 to impress one with the belief that it belongs to the genus Unio, as un- 

 derstood in its wider signification. It certainly lias an external ligament 

 exactly as we see in that genus ; while one of the casts shows the im- 

 pression of a single compressed, oblique cardinal tooth in the right 

 valve just over the scar of the anterior adductor, and near the margin. 

 The anterior adductor scar is rather deep and also near the margin. I 

 have not, however, seen the small scar placed just behind that of the 

 anterior adductor of Unio, in the internal casts of this shell. So far as 

 can be made out from the casts, there would seem to have been no lateral 

 teeth, but the specimens are not in a condition to warrant a positive 

 opinion on this point ; nor do they show the nature of the pallia! line. 



Locality and position: Dakota Group, or No. 1, of the Upper Mis- 

 souri cretaceous, opposite Sioux City, on the Missouri, in Dakota County, 

 Nebraska, where it occurs associated with Gyrena arenaria. 



ARC A? PARALLEL A. (MEEK.) 



Shell small, longitudinally oblong, being about twice and a half as 

 long as high, moderately convex ; cardinal and pallial margins straight 

 and nearly parallel ; anterior side short and rounding up regularly from 

 below and intersecting the cardinal margin at an obtuse angle above ; 

 posterior side long, a little wider than the other, with its margin com- 

 pressed and obliquely truncated above, but rounded below ; beaks de- 

 pressed, somewhat flattened, incurved, not very remote, and placed 

 about one-fifth the length of the valves from the anterior margin ; car- 

 dinal area very narrow, and apparently smooth ; muscular and pallial 

 impressions very obscure ; hinge with denticles longest posteriorly, where 

 they are directed upward and backward at. an angle of about forty- five 

 degrees to the cardinal margin ; from the posterior side they diminish 

 rather rapidly in size and length forward, so that they become very 



