Mesozoic and Ccenozoic Geology and Faloiontologij. 185 



Dijsganus encaitstus, D. haydenanus, D. bicarinatus, D. ^:)eiV/«?i'M.9; 

 DicloniKS pentagomis, D. perangulatus, B. calamariiis^ Monoclonius 

 crassus, Paronychodon lacitstris, Compsemysimhricarius, C. variolosus, 

 Polythorax missouriensis, Hedi'onchus sternbergi, Ceratodus eruci- 

 ferus, C. hieroglyphus^ Myledaplms bipartitus, Lcelaps hazenaniis, L. 

 loivifrons, Zapsalis abradens, Channpsosawms profundus, C. annectens, 

 C. brevicoUis^ C. vaccinsulensis, Scaphei^peton excisum, S. favosum, 

 S. laticoUe, S. tectum, and Hemitrypus jordananus ; and from the 

 Fox Hills Groiip, of Montana, JJronautus cetiformis. 



Prof O. C. Marsh* described, from the upper Cretaceous of Western 

 Kansas, Ichthyornis victor, Hesperornis gracilis, Lestornis crassipes, 

 Pteranodon comptus, P. ingens, P. longiceps, P. occidentalism P. velox, 

 and P. gracilis, now Nyctosaurus gracilis. 



In 1877, Arnold Haguef estimated the thickness of the Cretaceous 

 on the outlying ridges and foot-hills, east of the Colorado range, as 

 follows: Dakota Group, 300 feet; Colorado Group, 1,000 feet; Fox 

 Hills Group, 1,500 feet; and Laramie Group, 1,500 feet. 



The Dakota beds are essentially a sandstone formation, and as they 

 are usually hard and compact, frequently almost a quartzite, they form 

 a well-defined horizon. Lying between the easily-eroded Jurassic 

 marls and clays below, and the overlying blue shales, clays and crumb- 

 ling rocks of the Colorado Group above, the Dakota beds are usualty a 

 conspicuous feature in the ridges, which form the foot-hills of the main 

 range. In approaching the mountains from the Great Plains, the 

 Dakota beds are especially prominent, as they form the outlying mem- 

 ber of the series of upturned sedjmentar}^ beds, which rise so abruptly 

 above the plain; for although the overlying Colorado group is perfectly 

 conformable, the}' never occur high up on the long ridges, which form 

 a sort of barrier between the level countr}' and the mountain region 

 beyond. 



The Colorado Group is used to represent the Fort Benton, Niobrara, 

 and Fort Pierre Groups. The Fort Benton Group is only exposed along 

 the base of the abrupt^ ridges, and consists of dark, plastic clays, at 

 times distinctly bedded, and frequently occurring as thinl^^-laminated 

 paper shales. The lower beds are always more or less arenaceous, with 

 interstratified beds of purer clay, while the upper beds sometimes carry 

 thin seams of argillaceous limestone, which, in man}- places, can not be 

 distino;uished from similar beds in the Niobrara. Alono- the Laramie 



=■= Am. Jour. Sci. and Arts, 3d Ser., vol. xi. 

 t Geo. Sur. 40th parallel. 



