44 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



almost exactly equal. In size and general appearance (especially some of the 

 less distinctly undulated specimens), it reminds one of /. nobilis, Minister, as 

 figured by Goldfus (in his Petref. Germ., pi. cix, figs. 4, a, b) ; but even these 

 smoother specimens still show concentric undulations not represented in the 

 figures of Minister's species, which also has rather decidedly unequal beaks, 

 and its anterior margins more sinuous and inflected under the same. 



Our figures represent a strongly undulated specimen, reduced to about 

 half the natural diameter. Its lower and posterior margins are broken away, 

 and the anterior margins inflected by accidental pressure, so that the side- 

 view does not give a very good idea of its outline, and makes the hinge 

 appear proportionally too long. The larger specimen, from which our meas- 

 urements were taken, shows the hinge to be quite short and the posterior 

 margins of the valves curved forward above, so as to meet it at a more obtuse 

 angle. This larger specimen also has its undulations proportionally broader 

 and .much less distinct; and the anterior margins of its valves descend 

 straight and vertically from the beaks, without being inflected, but meeting 

 at an obtuse, though salient, angle. It likewise shows the radiating markings 

 rather more distinctly than the figured specimen, on which, though clearly 

 traceable, they are so feebly marked that they escaped Mr. Elliott's attention 

 in making the drawing. 



Locality and position. — Near Medicine Bow station, Wyoming Terri- 

 tory ; in beds equivalent to the Fort Pierre group of the Upper Missouri 

 Cretaceous series. 



Subgenus VOLVICERAMUS. 

 Inoccrainiis uuiboiiiitiis, M. & H. 



Plate 3, figs. 1, a, b, c; and pi. 4, figs. 1, a, b, and 2, «, b. 



Tnoceramus umbonatus, Meek and Haydeu, March (1858), Proceed. Acad. Nat. Sci. Philad., 50. 

 Compare I. involutus, Sowerby (1828), Miu. Conch., VI, llii), pi. 583. 



Shell attaining a rather large size, vertically subovate, extremely inequi- 

 valve ; height more than one-third greater than the antero-posterior diam- 

 eter ; base regularly rounded; hinge and interior unknown. Left valve very 

 convex; beak greatly elevated, gibbous, strongly and somewhat obliquely 

 involute, so as to form one and a half to two entire turns, the point termi- 

 nating near the anterior side; surface unknown, lliat of internal casts some- 

 times showing faint traces of concentric undulations. Right valve subcir- 



