INTRODUCTORY REMARKS. KLV 



In making comparisons for the determination of the equivalents of the 

 subdivisions of our Upper Missouri Cretaceous scries with those of Europe, 

 it is necessary to view i he Fori Benton and Niobrara groups, and possibly 

 even the Dakota also, as forming together one division ot the Cretaceous 

 system, because parallels of the smaller subdivisions cannot,. at least in our 

 present knowledge of the subject, be traced out in detail on opposite sides 

 of the Atlantic. The evidence that at least the Fort Benton and Niobrara 

 groups represent the Lower or Gray Chalk, and possibly also in part the 

 Upper Green-sand of the English geologists, will be readily understood by 

 the following list of Upper Missouri species from the groups mentioned, either 

 identical with, cr very closely allied to, species found at the horizons referred 

 to in England and on the continent. 



SPECIES FROM THE FORT BENT! >N AM) NIOBRARA SPECIES PROM THE LOWER CHALK AND UPPER 



GROUPS ( IF THE UPPER MISSOURI. GREKN-SAND OF EUROPE. 



Mortoniceras Shoslwnense, M Represented by M.vespertinus, Morton (sp.).* 



Prionoeyclus IFoolgari, Man tell (sp.) = Ammonites Woolgari, Mautell. 



Scaphites Warrenamts , M. & H.,verg nearly related to.. .S. cequalis, Sowerby.t 



Scaphites larvceformis., M. & H., same type as S. cequalis, Sowerby. 



Nautilus elegatis, Sowerby N. elegans, Sowerby. 



Inoceramus probhmaticus, Schloth I.problematicu8,Sch.i 



Inoceramus tutus, Mantell /. latus, Mant. 



The foregoing list of identical or very closely-allied species might be 

 extended by including forms found in equivalent beds of these groups farther 

 southward, while the evidence that these divisions represent the European 

 members of the Cretaceous suggested, receives further support from the fact 

 that we as yet know of no foreign strictly Upper Chalk species occurring in 

 them, and of but one marked form (our Inoceramus umbonatus of the Fort 

 Benton group) that is most nearly allied to any European Upper Chalk species.§ 

 Yet, in the succeeding rocks above, as shown farther on, the White Chalk 

 fauna is clearly represented by a number of closely-allied, and some identical 

 forms. 



As before, however, these upper divisions — the Fort Pierre and Fox 

 Hills groups — are so intimately related, palseontologically, that in comparing 



* Tbis is Ammonites Texanus, Roemer, which lias, according to Mr. Gabb, been found to be exactly 

 identical with the type of Dr. Morton's previously-published A. vesperiinus, the type of the genus 

 Mortoniecras. It has been found at Gossan, France, by Hauer, where it is believed to occur in beds of the 

 age of the Lower or Gray C'balk. 



tin Agassiz and Desor's German translation of Sowerby's Mineral G'ouchology, this is given as a 

 Chalk-Marl ( = Gray-Chalk) species. It is now regarded as an Upper Green-sand species by European 

 authors. 



{This species is said to occur in the Upper Green-sand, but is generally confined to the Gray Chalk. 



j This species is most nearly related to Inoceramus involutes, Sowerby, from the Upper Chalk, 



