XXXVI INTRODUCTORY REMARKS. 



facts, however, especially when we take into consideration the change of 

 sediments at or near where we have always placed the line between these 

 two rocks, seem to mark this as about the horizon where we find evidences 

 of the most marked change of physical conditions. 



Among the most common or more conspicuous fossils of this rock may 

 be mentioned Cucullcea Shumardi, Axincea subimbricata, Veniella humerosa 

 Pyropsis Bairdi, Fusus Dakotensis, Baculites grandis, Placenticeras lenticularis, 

 Scaphites Conradi, S. NicoUetii, 8. Mandanensis, S. Cheyennensis, Nautilus 

 Dekayi, Mosasaurus Missouriensis, etc. The fossils of this group are figured 

 on our plates 28 to 36, inclusive. 



The typical and most characteristic localities of this division of the Upper 

 Missouri Cretaceous series are at Fox Hills, between Cheyenne and Moreau 

 Rivers, above Fort Pierre, Dakota; it also extends across the Missouri in the 

 region of Long Lake, at both of which localities it contains numerous speci- 

 mens of its characteristic fossils, usually in a fine condition of preservation. 

 It is likewise seen immediately underlying the Tertiary bone-beds of the 

 Mauvaises Terres, on Sage Creek and near there, but here it seems to be 

 nearly barren of fossils. 



On the Upper Missouri, underlying the Brackish-water Lignite beds at 

 and near the mouth of Judith River, some fossiliferous Cretaceous beds occur 

 that we were for a long time in doubt where to place in the section. More 

 recently, these have been elsewhere discovered connected in such a way with 

 the Fox Hills beds, and containing so many of the common fossils of the 

 same, as to show that they form an upper member of that group. The fos- 

 sils from this horizon are figured together on our plates 37, 38, and 39. Of 

 these forms, the most conspicuous are Cardlum spetiosum, lnoceramus pertenuw, 

 Tancredia Americana, Mactra alta, M. formosa, Pholadomya subventricosa, 

 Gly timer h occidentalism etc. 



On the north, the Fox Hills group, like the Fort Pierre, extends into 

 the British possessions, a few of its fossils having been identified by the 

 writer among Professor Hind's collections from the south branch of the 

 Saskatchewan, where they occur in a green sand matrix. 



At the Black Hills, and along the mountain-ranges west of there, the 

 Fox Hills group is seen in places well developed. P also occurs at Deer 

 Creek on the North Platte, and at various places along the eastern base of 

 the Rocky Mountains; the upper beds of the group (already mentioned 



