INTRODUCTORY REMARKS. XXXI 



ceed i rig one above, as wc ascend the Missouri, from the outcrops of the Dakota 

 group near the mouth of Big Sioux River. 



As developed along the Missouri at the typical localities, this group or 

 division is a heavy-bedded pure chalk-marl, being almost entirely composed 

 of calcareous matter, not generally harder or more compact than chalk. In 

 fresh exposures, it presents a bluish or lead-gray color; but, when weathered, 

 it becomes whitish or ash-colored, or often tinged with yellow. Some of 

 the lower beds, however, at places pass into a whitish or light-gray limestone, 

 easily splitting into irregular thin pieces. In the district mentioned along the 

 Missouri, the maximum thickness of this division has been roughly estimated 

 at about two hundred feet. 



The most common fossils of this group here are Ostrea congesta, Inoce- 

 ramus problematic/is, fragments of a small compressed Baculite, and large 

 scales of cycloid fishes. The little oyster is quite abundant at places, and 

 nearly always found attached to pieces of the outer fibrous layer of a large 

 Inoceramus, probably belonging to the species figured by the writer in Mr. 

 King's report, under the name /. deformis. The specimens of Inoceramus 

 problematicus are usually most abundant in the harder layers that sometimes 

 assume the character of a whitish limestone, in which the much more rarely 

 seen little Baculite also occurs. The fossils from this group are illustrated 

 on our plate 9. 



Although these most common fossils also occur in the Fort Benton group 

 below, the latter rock contains a number of molluscan remains and several 

 species of fishes and some Saurians, not known to ascend into the group under 

 consideration. Again, the Niobrara group contains the remains of several 

 species of birds, first made known by Professor Marsh, others of Plerodactylus, 

 and other vertebrates described by Professor Marsh and Professor Cope, that 

 are not yet known in the Fort Benton group. Professor Cope has identified 

 from the Niobrara group Ptychodus Mortoni and P. polygyrus, Agassiz, which 

 are also said to occur in Alabama. 



It is worthy of remark that the pearlaceous inner layer of the shells of 

 Inoceramus (and of other types provided with this layer) seems never to be 

 preserved in this formation, and rarely in the next below, only the outer 

 fibrous portion being present ; while shells of other kinds, excepting those 

 belonging to the Ostreidce, are, in nearly all cases, found only represented by 

 casts. In this respect, the fossils from these lower beds contrast strongly with 

 those from the later Cretaceous deposits of these western districts. 



