formations of the Middle Cretaceous. 317 



This fact cannot be regarded of mere local importance, for it 

 has, for a long time, been recognized over the entire West, 

 wherever the summit of the formation lies exposed. Among 

 these forms are Mytilus subarcuahis, Crenella elegantula, 

 Nucida cancellata, Cardium \Ethmocardixtn%\ speciosum* 

 Solemya subplicata, Sphmriola cordata, Veniella humilis* 

 Callista Deweyi, Callista [Dosinopsis~\ Owenana, Mactra 

 alta* Tellina scitula, Tancredia Americana* Liopistha 

 \CymellcC\ undata, Fasciolaria Cheyennensis, Pyrula Bairdi, 

 Fusus sp. f Psettdohuccinum JVebrascense, Anehura Amer- 

 icana, Turritella sp. f Dentalium sp. ?* Cylichna sp. ?*, etc., 

 etc. According to Dr. C. A. White, such of the above as are 

 starred especially characterize this horizon ; the others may 

 range from this, lower. In plant life Halymenites major is 

 generally met with on all sides. 



From the foregoing details, the following relations of the 

 several formations to each other may be clearly and legitimately 

 deduced : first, the component strata of the Fort Benton and 

 Niobrara frequently, and the life generally, shade into each 

 other ; second, this is again, even more forcibly, paralleled in 

 the sedimentation and life of the Fort Pierre, and Fox Hills ; 

 third, — and on the contrary, — between the Niobrara and Fort 

 Pierre, or, which is the same thing, between the Colorado and 

 Montana groups (as was suggested they should be designated 

 early in this paper), there is a differentiation of both sediments 

 and life, greatly in excess of any similarities in these respects 

 that may be noticed from time to time in any particular 

 locality. These facts — especially the ones in regard to life — 

 are considered by Dr. White, the eminent authority upon the 

 North American Cretaceous, as furnishing a most satisfactory 

 basis for the grouping of the several formations as here 

 adopted ; and, indeed, it is on account of the paleontological 

 relations of the formations to each other, that he himself was 

 led, as early as in 1876 and '77, to employ a precisely similar 

 system of grouping to that here suggested, though under a 

 partially different nomenclature. 



The above distinctions were also remarked upon by Professor 

 Meek in his work — "Invertebrate Palaeontology," vol. ix, U. S. 

 Geological Survey of the Territories, 1876, pp. xxxii and xxxiii, 

 — where he says : " In passing from this formation [the Niobrara] 

 to the next above, we cross the most strongly-marked paleonto- 

 logical break in the whole series, unless that between the 

 Dakota group and Fort Benton group may be equally so. As 

 far as yet known, none of the Dakota group species occur in 

 the beds above, but then the number of species yet found in 

 that division and the Fort Benton group is hardly sufficient to 

 warrant the conclusion that some forms mav not be common to 



