166 THE WASATCH AND BRIDGER FAUNJE. 



left its remains in the Bridger beds, judging from vertobrnc wliicli I found 

 on Black's Fork of Green River. 



A somewliat similar'sniall species is found in the Wasatch beds of New 

 Mexico, the C. chamensis Cope. In that species the dermal scuta are articu- 

 lated together by suture. 



MAMMALIA. 



The lacustrine Eocene strata have been found in all parts of the world, 

 where existing, to contain remains of an abundant mammalian life. The 

 character of this mammalian fauna has been found to be particularly inter- 

 esting, and for the following reasons. 



Much light is thrown on the history of the Mammalia by the researches 

 into the structure of those of the Eocene formation, and I deem it demon- 

 strated to a certainty that the case with the mammals of this formation is 

 the same as with the rej)tiles of the Permian, i. e , that the family types 

 are all more generalized, and the orders not nearly so widely distinguished 

 as in later periods of the world's history. 



The recent orders of fishes were in existence in the Cretaceous period, 

 and probably earlier. Their period of evolution was in the Devonian and 

 the Carboniferous periods. The existing orders of reptiles were all estab- 

 lished prior to the Eocene; the period of evolution was the three Mesozoic 

 ages, but especially the Permian. The orders of birds were inchoate in 

 the Cretaceous, but when they were fully differentiated is unknown. The 

 existing orders of Mammalia were already established in the Miocene period; 

 during the Eocene they were in process of differentiation, and were less, or 

 scarcely at all distinctly defined.* 



Tiie characters of the Placental Mammalian orders which existed 



during Eocene time are as follows: 



1. Ungual phalaiiffcs cla\v.s (uuguiculate). 



a Cerebral heniispberes small ; cerebellum aud olfactory lobes large 



anil iincovcrfd. 

 fi Teeth slu-atht'd in t-namt'l. 



Glenoid cavity longitudinal ; mandibular condj'le round ; anterior 



limbs ambulatory Rodentia. 



•Sci' Aiimml Report U. S. Gcol. Surv. Terra. 1872, \k G4,'>. 



