72 Cretaceous. 



alis, and from the great bend of the Missouri river below Fort Pierre, 

 Ostrea inornata. 



From (Niobrara Group) near the mouth of the Niobrara river, An- 

 omia obliqua ; from (Dakota Group) near the mouth of the Big 

 Sioux river, Mactra Siouxensis. F. B. Meek described, from near 

 Bear river, on Sulphur Creek, Anomia concenti'lca, Corhula concentri- 

 ca, C. engelmanni, C.pyriformis^ and Melania humerosa, now Pyrgu- 

 Ufera humerosa; from the North Platte, Inoceramus simpsoni ; from 

 Ham's Fork, northeast of Fort 'Qv\([gQ\\ Melampus priscus, wow Rhy- 

 tophorus priscus, Melania simpsoni. now Goniobasis simpsoni, M. 

 arcta, 31. nitidula, now Limnosa nitidula, L. similis, L. vetusta, 

 Planorbis spectabilis, P. tctahensis, and from near Fort Bridger, Unio 

 haydeni. Some of the latter species probably belong to the Lower 

 Eocene. 



In 1861, Meek and Ha\'den,* as before mentioned, separated the 

 Cretaceous rocks of the Missouri region into five subdivisions, in 

 ascending order, as follows : 



1. Dakota Group, consisting of yellowish, reddish, and occasionally 

 white sandstone, with, at places, alternations of various colored clays 

 and beds and seams of impure lignite ; also silicified wood, and great 

 numbers of leaves of the higher types of dicotyledonous trees, with 

 casts of Pharella dakotensis, Axinaea sioiixe7isis, and Cyprina arenarea. 

 Pound at the hills back ot the town of Dakota; also extensively de- 

 veloped in the surrounding country in Dakota county, below the mouth 

 of Big Sioux river, thence extending southward into northeastern 

 Kansas and beyond. Estimated thickness, 400 feet. 



2. Fort Benton Group, consisting of dark gra}^, laminated clays, 

 sometimes alternating near the upper part with seams and la^-ers of 

 soft gra}^ and light colored limestone, Inoceramus problematicus, I. 

 tenuirostratus, I. latus, I.fragilis, Ostrea congesta, Venilia mortoni, 

 Pholadomya papyracea, Ammo7iites miillani^ A. percarinotits, A. ves- 

 pertinus, Scaphites warreni^ S. larvoiformis^ 8. ventricosiis, S. vermi- 

 formis, Nautilus elegans, etc. Extensively developed near Fort Ben- 

 ton, on the Upper Missouri; also along the latter from ten miles above 

 James river to Big Sioux river, and along the eastern slope of the 

 Rocky Mountains as well as at the Black Hills. Estimated thickness, 

 800 feet. 



3. Niobrara Group, consisting of lead-gray calcareous marl, weather- 

 ing to a yellowish or whitish chalky appearance above, containing 



* Proc. Acad. Nat. Sci. 



