46 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



edge is a shell described by Dr. Slmmard from the Cretaceous rocks of 

 Texas, under the name of I. capulus (Trans. Saint Louis Acad. Sci., iv, 

 606). I only know the form described by Dr. Shumard from his descrip- 

 tion, not having seen figures or specimens of it; but judging from his 

 description, I think he is right in considering it a distinct species, since it is 

 marked by small, distinct, concentric undulations, while the corresponding 

 valve of our Upper Missouri species is almost entirely smooth. He also 

 mentions traces of radiating ribs on the umbo, of which I have seen no indi- 

 cations on any of our Upper Missouri specimens. 



From the same locality and position as those from which the species 

 under consideration was collected, Lieutenant Mullan's party obtained another 

 somewhat analogous species, which we have described in the Proceed. Acad. 

 Nat. Sci. Philad., under the name of I. exogyroidcs. Like /. umbonatus, it 

 has a very gibbous left valve, and probably a nearly flat right valve ; but it 

 will be readily distinguished by the much more depressed and oblique umbo 

 of its left valve. This depression of the umbo gives it a nearly circular, 

 instead of a vertically oval, outline. 



Locality and position. — Twenty miles below Fort Benton, on the Upper 

 Missouri; from the Fort Benton group, or No. 2 of the Cretaceous series. 



I ii o c e r a ni ii s exogyroidcs, M. & H. 



Plate 5, figs. 3, a, b, c. 

 Inoceramus exogyroidee, Meek and Hayden (1862), Proceed. Acad. Nat. Sci. Philad., 26. 



Shell rather large ; left valve suborbicular, its height being a little greater 

 than its length from the anterior to the posterior side, very gibbous ; anterior 

 and posterior sides rounded, and forming with the base about three-fourths 

 of a circle, the posterior curve being broader than the other ; cardinal margin 

 comparatively short, and apparently a little arched ; beak large, elevated, 

 gibbous, distinctly incurved and directed obliquely forward, so as to bring its 

 point near the anterior margin ; surface of cast smooth, or marked by obscure 

 concentric undulations. (Right valve unknown.) 



Length from anterior to posterior margin, 5 inches ; height, 5.50 inches ; 

 convexity, near 3 inches. 



No right valves of this species have yet been found; but, judging from 

 the gibbous diameter, and involuted beak of the left valve, it is probable that 

 the right will be found to be much more compressed, so as to make the 



