stanton.] GEOGRAPHIC DISTRIBUTION. 1!> 



scribed by Meek and others from the typical Fort Benton and Niobrara 

 of the Upper Missouri section and from their undoubted equivalents 

 in the Black lulls and along the Front range in Colorado Lave been 

 taken as the nucleus of the fauna. To those have boon added others 

 from the same beds and those from other regions that arc associated 

 with species believed to be characteristic of tins horizon. Some fortu 

 nate discoveries of an unusually abundant and varied fauna in the Fort 

 Benton of southern Colorado, and the careful comparison of collections 

 from many localities, thus correlating the bods stop by stop, have added 

 a considerable number of species to the fauna, some of them new and 

 many more thai have either been assigned to a higher horizon or 

 described simply as (Vetacoons without reference to any of its divisions. 

 As a necessary result of recently ascertained facts concerning the ver- 

 tical distribution of some of these species, certain strata that have 

 usually been referred to the Montana formation in Utah and adjoining 

 Slates are now transferred to the Colorado. This subject will be dis- 

 cussed somewhat in detail when the typical sections arc described. 



The Colorado formation has been recognized by means of its charac- 

 teristic fossils in Iowa, Minnesota, the Dakotas, Nebraska. Kansas, 

 Colorado, Wyoming, Montana, Utah, Arizona, and New Mexico. It is 

 also well established that equivalent strata exist- in Texas ami (lie 



adjacent regions and over large areas in British America, In Texas 

 the Eagle Ford shales and the Austin limestone are evidently the 

 equivalent of the Fort Benton ami the Niobrara, respectively, but their 

 fauna is in some respects quite different from that of the typical Colo- 

 rado and for that reason its peculiar species have been excluded, 

 though many of those hero described are known to occur in both 

 regions, and it is probable that more, thorough collecting will add many 

 to the common list. 



The species described from British America have been excluded be- 

 cause the geologists have often found difficulty there in discriminating 

 between the Colorado and Montana formations, and consequently a 

 number of the described upper Cretaceous species have not been as- 

 signed to either horizon. While many species evidently belong to the 

 Colorado fauna, there are many others that are doubtful and must re- 

 main so until more is known of their associates and their Stratigraphic 

 position. 1 For similar reasons a few known species from portions of 

 our area, have been omitted, some of which probably belong to this 

 fauna, 



From southern IMahand from New Mexico a low species are included 

 whose stratigraphic position is not definitely known, but they are 

 usually associated at the same locality with known Colorado species. 



The area to which the Colorado formation is thus arbitrarily re 



1 Since t lie alios p. W as \\ i it ten M r. .1 . F. W hitc;i ves lins published defeOriptioUS Of (our new species 



(»r Ammonites From the I retaceous rocks of Athabasca (Trans. Roy. Soc. Canada, see. iv. 1802, pp. 

 ill l lit) n ml referred them t<> the horizon of the ( lolorado formation, i ln\ belong to genera .thai have 

 nol j et l >ee u i (mi in I in 1 1 mi formation in i he United States and are suggestive of an older horison. 



