Cope.] 1"" fDec. 16, 



a mandible of an adult animal in good preservation. In their robust 

 character the premolar teeth resemble those of the M. turgiclus, but are 

 not relatively so large, nor is the last true molar relatively so small, as in 

 that species. The heel of the third premolar is obsolete, and that of the 

 fourth is a wide cingulum. Neither exhibit an anterior basal tubercle, 

 and in both the principal cusp is stout. The true molars widen posterior- 

 ly to the anterior part of the last molar. The latter contracts rapidly to a 

 narrow heel. The tubercles are all subconic, and the median ones of the 

 last molar are small. There are no cingula, and the enamel is smooth. 



The ramus is not robust, and is of moderate depth. Its inferior border 

 rises below the middle of the last molar tooth, and posteriorly. There is 

 a "mental" foramen below the contact of the fourth premolor and first 

 true molar. 



Measurements. M. 



Length of bases of six posterior molars 047 



" " three premolars... 024 



P^m. II .009 



P-m. IV....... 008 



P-m. IV 005 



Diameters basis of M. II j anteroposterior 0075 



t transverse 0070 



Diameter basis M. Ill { anteroposterior 0084 



t- transverse. 0070 



Depth of ramus at P-m. II 0080 



M. II 0140 



This species is named from the Crow Indian name of the Big-Horn 

 river, Etsagie, 



Concluding Remarks. 

 The paleontologist who has examined the preceeding list, will readily 

 perceive that it represents fully the Wasatach fauna, with little admixture 

 of earlier or later forms. The only genus which belongs to the Bridger or 

 middle Eocene, which occurs in the Big-Horn basin, is PappicMhys, The 

 characteristic Bridger genera Eyrachyus, Palceosyops, Uintatherium, and 

 the Tillodonta, are absent, and their place is taken by Phenacodus, Syra- 

 cotherium, Goryphodon and Twniodonta, as in New Mexico. Several genera 

 are, as elsewhere, common to the two horizons, and two species cannot be 

 distinguished in the parts preserved. Such as Eyopsodus paulus and 

 E. vicarius. A closer comparison may be made with the Wind- 

 River group, on which I published a report in the Bulletin of the U. S. 

 Geological . Survey of the Territories. * The following genera found in 

 that formation have not been obtained from the Big-Horn . Protopsalis, 

 Lambdotherium, Palceosyops, Hyrachyus.\ Genera of the Big-Horn not 

 obtained from the Wind-River : Cynodontomys, Anaptomorphus ; Mesonyx, 



*1881, Feb. p. 201. 



f Since making my report on the Wind-River fauna, I have found the anterior 

 part of the lower jaw of a species of this genus. 



