430 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



on our plate 25, little is found to distinguish it from the same, excepting its 

 very ventricose form, a character not alone generally reliable for distinguish- 

 ing species in this and allied groups of shells. Its septa will be observed to 

 agree almost exactly, excepting in mere minute individual details, with those 

 of the variety brevis, figured on plate 25, and to present the same differences 

 in the smaller lobes from the variety quadrangularis, that the variety brevis 

 does. At one time I was rather inclined to think that this great difference 

 in the convexity of these shells might be merely sexual ; but the fact that the 

 variety or species plenus was only found at a locality on Yellowstone River 

 (where it occurs associated with some of the more compressed forms), while 

 no specimens of it were found at numerous other localities where the latter 

 forms are common, does not seem to sustain this view. 



Locality and jiosition. — Yellowstone River, 150 miles from its mouth ; 

 in the upper part of the Fort Pierre group of the Upper Missouri Creta- 

 ceous series. The splendid specimen figured on our plate 26 was discov- 

 ered by Lieutenant (now General) G. K. Warren, of the United States 

 Topographical Engineers. 



Subgenus DISCOSCAPHITES. 



[Section a. J 



Scapliites Conradi, Morton (sp.) 

 Plate 36, figs. 2, a-c. 



Ammonites Conradi, Morton (1834), Synop. Org. Remains Oct. Group U. S., 39, pi. 16, figs. 1,2, and 3 (not 



fig. 4, pi. xix). 

 Scapldtes Conradi, d'Orbigny (1850), Prodr. de Paleont., II, 214.— Meek and Hayden (1856), Proceed. 



Acad. Nat. Sci. Pliilad., 284.— Gabb (1861), Syuop. Moll. Cret., 32 (not S. Conradi, von 



Buch (1849), Bull. G<5ol. Soc. Fr., VI, 2e ser.) 

 ? Scaphites pnlvherrimus, Roomer (1841), Verst. norddeutscben Kreidegeb., 91. 

 Ammonites Danes, d'Orbigny (1850), Prodr. de Pale'ont., 11,213. 

 Ammonites Nebrascensis, Owen (1852), Report Iowa, Wisconsin, and Minnesota, 577, pi. 8, fig. 3; and 



pi. 9, fig. 2. 



Shell short-oval-subdiscoid or subcircular in outline, and rather strongly 

 compressed, often attaining a very large size; section of volutions oval, being 

 higher than wide ; inner turns closely involute and deeply embracing, gener- 

 ally nearly rounded on the periphery ; umbilicus small; deflected part of 

 outer volution very short, and scarcely, or not at all free at the aperture, 

 which is oval, or with inner side more or less sinuous ; surface ornamented 

 with moderate-sized, straight, or sometimes slightly arched costse, some of 

 which bifurcate once or twice, while shorter ones are occasionally intercalated 

 between the others ; costse all passing nearly straight across the periphery, 



