OF WESTERN NORTH AMERICA. 



61 



The archaic mammals are now represented only by the true Hyseno- 

 dontinse, which are probably of European and African origin. Among 

 Marsupialia, Didelphyidae are somewhat doubfully recorded in this 

 stage. 



Of the modernized mammals, among Rodentia (a) Ischyromyidse, 

 represented by IscJiyromys; Paramijs disappears or gives rise to Sci- 

 urus; and there first appear the modern (b) Leporidae, (c) Castoridse, 

 (c^) Sciuridse, and (e) Geomyidae. Of Insectivora the Leptictidse 

 continue, as well as animals analogous in dentition to Centetes and 

 Solenodon. The Carnivora are thoroughly modernized by the appear- 

 ance of true Canidse {Cynodictis , DapTisenus), Mustelidse, and Felidse 

 (Machserodontinse). Eight families are known of Ungulata-Perisso- 

 dactyla, including 6 Eocene families which survive from the upper 

 Eocene, namely, (a) Equidae, (b) Tapiridae, (c) Amynodontidae, (d) 

 Ilyracodontidae, (e) Lophiodontidae, (/) Titanotheriidae,^ which reach 

 the climax of their evolution and suddenly disappear, (g) the aberrant 

 Perissodactyla-Chalicotheriidae are first positively recognized, (h) the 

 Rhinocerotidae, ancestors of Diceratherium, and another subfamily 

 ( ?Aceratheriinae) also first appear. Of Artiodactyla 6 families occur, 

 as follows: Three previously known Eocene families, (a) Oreodontidae, 

 (h) Camelidae, and (c) Hyper tragulidae, continue; (d) the Dicotylidae 

 first appear, either of American origin from the Great Plains or of 

 Eurasiatic origin as a side branch of the Suoidea; (e) the Anthraco- 

 theriidae also first appear, probably by migration from Europe, and 

 are represented by Hyopotamus in the Chadron formation; (/) the 

 bunodont Achaenodontinae of the upper zones in the Washakie and 

 Uinta basins are succeeded or replaced by the Elotheriinae orEnte- 

 lodontinae closely allied to the European Entelodon. 



Monument Creek formation. — The following description of this for- 

 mation is taken from a paper by Darton published in 1906:^ 



On the high divide between the Platte and Arkansas drainage basins, at the foot 

 of the Rocky Mountains, there is an extensive deposit of conglomerates, sand, sand- 

 stone, gravel, and clay, known as the Monument Creek formation. It lies on the 

 Laramie formation to the east and the Arapahoe formation to the west, and at Palmer 

 Lake it abuts against the granite at the foot of the mountain. There are two members, 

 a lower one of sands and clays and an upper one of conglomerate and sandstone. The 

 latter caps numerous buttes and plateaus in the high region west and north of Calhan 

 and north of Monument. 



Fossil bones of Titanotherium have been discovered by the writer c and Mr. C. A. 

 Fisher in the upper member in the region north of Calhan and southwest of Elizabeth, 

 which indicate that this portion of the formation is of Oligocene age. The lower 

 member may be Oligocene, or perhaps Wasatch or Bridger, in age. 



oQsborn, H. F., The four phyla of Oligocene titanotheres: Bull. Am. Mus. Nat. Hist., vol. 10, 1902, 

 pp. 91-109. 



b Darton, N. H., Geology and underground waters of the-Arkansas Valley in eastern Colorado: Prof. 

 Paper U. S. Geol. Survey No. 52, 190G. 

 c Darton, N. H., Age of Monument Creek formation: Am. Jour. Sci., 4th ser. vol. 20, 1905, pp. 178-180. 



