522 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



truncated below; basal margin forming a semi-oval, or semicircular curve, 

 the most prominent part of which is generally near the middle; beaks rather 

 elevated, gibbous, and very nearly central; surface merely marked by lines 

 of growth, without regular furrows ; pallial line with a small, shallow, 

 rounded, or subangular sinus. 



Length and height of a medium-sized specimen, each 1 inch; convexity 

 of same, 0.71 inch. Some imperfect specimens seem to have attained nearly 

 twice the size of that from which these measurements were taken. 



This is a remarkably thick, strong shell, especially in the umbonal 

 regions. None of the specimens that I have seen show its hinge; but, judg- 

 ing from its association with fresh- and brackish-water types, we have little 

 reason to doubt that it belongs either to the genus Corbicula or Cyrena, more 

 probably to the former. 



The specimen from which our figure 6, b, was drawn has the valves 

 accidentally compressed below, so as to give it an unnaturally cuneate appear- 

 ance. 



Locality and position. — Bad Lauds of the Judith River, Montana, on the 

 Upper Missouri River : probably belonging to the Upper Cretaceous. 



Corbicula Sfebrascensis, M. & H. 



Plate 43, figs. 2, a, o (not 2, c). 



Cyrena intermedia, Meek aud Hayden (1856), Proceed. Acad. Nat. Sci. Pbilad., VIII, 116. (Not Cyrena 



[Corhieula] intermedin, Mellville, 1S43.) 

 Corbicula Ifebraxcensis, Meek and Hayden (18G0), Proceed. Acad. Nat. Sci. Pbilad., XII, 423. 



Shell ovate, moderately compressed, rather thin ; extremities rounded ; 

 base semi-elliptical or semi-ovate, the most prominent part being usually in 

 front of the middle; cardinal margin rather thin, forming an angle of about 

 110° at the beaks, the anterior slope being slightly concave in outline, and 

 more abrupt' than the posterior, which forms a broad, convex, descending 

 curve ; beaks of medium height, rather small, and located a little in advance 

 of the middle; surface marked by fine lines of growth, and sometimes having 

 a few somewhat stronger concentric furrows. 



Length, 0.77 inch; height, 0.63 inch; convexity, 0.34 inch. 



This species agrees with our C. cytheriformis nearly in general form, 

 excepting that it is more compressed. It is also a smaller, proportionally 

 thinner shell. 



Locality and position. — Moreau River. Dakota; from the horizon of the 

 Judith River group, or the base of the Fort Union group. 



