white.] CATALOGUES OF FOSSILS. 265 



gin of all of them were substantially the same, and yet, so far as any 

 accumulated evidence shows, those mammalia were not preceded in the 

 Laramie period by any related forms. Such suddenness of introduction 

 makes it almost certain that it was caused by the removal of some phys- 

 ical barrier, so that ground which was before potentially Tertiary be- 

 came so by actual faunal occupancy. In other words, it seems certain 

 that those Tertiary mammalian types were evolved hi some other region 

 before the close of the Laramie period, where they existed contemporane- 

 ously with at least the later Laramie Dinosaurians of Cretaceous types, 

 and that the barrier which separated the fauna} was removed by some 

 one of the various movements connected with the evolution of the con- 

 tinent. The climate and other physical condition s which were essential 

 to the existence of the Dinosaurians of the Laramie period having evi- 

 dently been continued into the Tertiary epochs that are represented by 

 the Wasatch, Green Eiver, and Bridger Groups, they might doubtless 

 have continued their existence through those epochs as well as through 

 the Laramie period, but for the irruption of the mammalian horde, to 

 which they probably soon succumbed in an unequal struggle for exist- 

 ence. 



CATALOGUES OF FOSSILS. 



The following lists of fossils are those of collections which have from 

 time to time been sent to the office of the survey from different places in 

 the western part of the national domain 'by persons who are not, or 

 were not then, officially connected with the survey. They are introduced 

 here partly to show the association of the species, a part of which were 

 originally described in publications of the survey, and partly to show 

 the geographical distribution of species and types, especially those of 

 Cretaceous age, in the strata of North America. 



LIST OF CRETACEOUS FOSSILS SENT BY MR. ARTHUR LAKES FROM 

 BEAR CREEK VALLEY, NEAR MORRISON, COLORADO. 



Pox Hills Group. 



1. Pteria linguiformis Evans & Shumard; 750 feet below the coal.* 



2. Pteria (Pseudoptera) fibrosa Meek & Hayden ; 750 feet below the coal. 



3. Inoceramus oblongus Meek; 750 feet below the coal. 



4. Cardium speciosum Meek & Hayden; 200 feet below the coal. 



5. Tellina scitula Meek & Hayden; 200 feet below the coal. 



6. Maetra holmesi Meek, sp.; 750 feet below the coal. This is the species 



which was originally described as Cyrena ? and supposed to have 

 belonged in Laramie strata. 



7. Pachymya herseyi White; 200 feet below the coal. 



8. Dentalium gracile Hall & Meek; 200 feet below the coal. 



9. Lunatia occidentalis Meek & Hayden; 750 feet below the coal. 



10. Baculites ovatus Say; 750 feet below the coal. 



11. Scaphites nodosus Owen; 750 feet below the coal. 



12. Scaphites mandanensis Morton?; 750 feet below the coal. This is 



recognized as belonging to the same species as the fragment which 

 was found associated with Maetra holmesi at the original locality 

 on Ealston Creek. (See remarks under ISTo. 6 of this list.) 



13. Placenticeras placenta Lekay (var.) ; 750 feet below the coal. 



*The coal referred to is within and near the base of the Laramie Group, hut all the 

 fossils of the list are found in unmistakable Cretaceous strata. 



