NATURAL SCIENCES OF PHILADELPHIA. 427 



v 

 fine state of preservation, the nacre of many of the shells being as brilliant 

 as when the animals were living. In this latter respect they present a marked 

 contrast with the dull appearance of those from the formations below. In most 

 cases they are found enveloped in very hard, dark argillo-calcareous concretions, 

 from a few inches to several feet in diameter. Those occurring in the lower 

 foasiliferous zone, at the base, are, Mosasaurus Missouriensis, Callianassa Dance-, 

 Ammonites cwnplexics, Baeulites ovatus and B. compressus, Hilicoceras Mortoni, H. 

 cochleatum, H. tortum, II. umbilicatum, Fusus viniculum, F. Shumardi, Buccinum 

 const ric lu in. Amawopsis paludinaformis, Anisomyon borcalis, Inoceramus sublcevis, 

 I. incurvus, &c. &c. 



In the upper fossiliferous zone, organic remains are more abundant than in 

 the lower ; the following list contains the names of many of those usually found 

 at this horizon, viz., bones of Mosasaurus Missouriensis, with Nautilus Dekayi, 

 Ammonites placenta, Scaphites nodosus, S. Nicolletii, Baeulites ovatus, B. compressus, 

 Apli/chus Cheyennensis, Fusus subturritus, F. ? tenuilineatus. Gladius? Cheyennensis, 

 Margarita Nebrascensis, Dentalium gracile, Tectura occidentalis, Anisomyon patel- 

 liformis, A. alveolus, Bulla Nebrascensis,] Xylophaga elegantula, Corbulamella gre- 

 garea, Cardium varum, Jjiicina occidentalis, Crassatella, Evansi, Modiola Mcehii, 

 Inoceramus convexus, I. Mortoni, I. Nebrascensis, I. Sagensis, I. Vanuxemi, &c. &c. 

 Several of these fossils pass up into the formation above. 



Fox Hills Beds. This formation is generally more arenaceous than the 

 Fort Pierre Group, and also diifers in presenting a more yellowish or ferrugi- 

 nous tinge. Towards the base it consists of sandy clays, but as we ascend to 

 the higher beds, we find the arenaceous matter increasing, so that at some 

 places the whole passes into a sandstone. It is not separated by any strongly 

 denned line of demarcation from the formation below, the change from the fine 

 clays of the latter to the more sandy material above, being usually very gradual. 

 Nor are these two formations distinguished by any abrupt change in the organic 

 remains, since several of the fossils occurring in the upper beds of the Fort 

 Pierre Group pass up into the Fox Hills Beds, while at some localities we find 

 a complete miogling in the same bed of the forms usually found at these two 

 horizons. Indeed, we might with almost equal propriety, on palaeontological 

 principles, carry the line separating these two formations down so as to include 

 the upper fossiliferous zone of the Fort Pierre Group, as we have defined it, ia 

 the formation above. All the facts, however, so far as our present information 

 goes, taking into consideration the change in the sediments at or near where 

 we have placed the line between these two rocks, seem to mark this as 

 about the horizon where we find evidences of the greatest break in the con- 

 tinuity of physical conditions. 



The formation under consideration is most distinctly marked at Fox Hills, 

 between Cheyenne and Moreau Rivers, above Fort Pierre ; and it also extends 

 across upon the opposite side of the Missouri, in the region of Long Lake. It 

 likewise immediately underlies the Tertiary bone beds of the Muuvaises Terres 

 at Sage Creek and other places in the vicinity of White River, but is not very 

 fossiliferous. so far as known, in that region. At a single locality on the Yel- 

 low Stone River, about one hundred and fifty miles from its mouth, there is a 

 low exposure having the lithological characters of the Fort Pierre Group, but 

 containing a complete mingling of the fossils usually found in the upper part 

 of that rock, with those generally occurring in this. 



On the Upper Missouri, in the region of Mussel Shell and Milk Rivers, the 

 Fox Hills Beds do not seem to be very well defined as a distinct rock. A few- 

 specimens, however, apparently from this horizon, were collected by Prof. 



t This species we had named B. occidentalis, in our paper of March, 1856. As that 

 name had previously been used for a recent species of this genus by A. Adams, it be- 

 comes necessary to give our shell another name, and we now propose to call it B. Ne- 

 brascensis. 



1861.] 



