54:0 



/ 



CENOZOIC EKA— AGE OF MAMMALS. 



potamus. Owen, indeed, made the Elephant, Mastodon, etc., a distinct 

 order, under the name of Proboscidians, but these are probably best 

 regarded as a very distinct offshoot or sub-order of the Perissodactyls. 



TAPIR RHIN? 



HIPPOPOT? HOG 



Fig. 921. 



PACHYDERMS<- 



PBJMAL.I UNGULATE 



(PHENACODUS.) 



-Diagram illustrating the Differentiation of the Different Families of Ungulates. 



Now in the Earliest Tertiary the sub-orders Artiodactyls and Peris- 

 sodactyls were united in a common ancestor or primal Ungulate, from 

 which they afterward separated. The Phenacodus of Cope (Fig. 909, 

 p. 530) seems to be such a primal Ungulate. Each of the primary 

 branches then divided and again divided, until the extreme branch in 

 one direction became the Horse, and the extreme branch in the other 

 direction the Ox. In the tree above we have attempted, in a general 

 way, to represent the differentiation of the several orders of Ungulates. 

 The Cuvierian orders, Pachyderms and Ruminants, are indicated by a 

 vinculum. It is seen at a glance why, in studying living animals alone, 

 the Ruminants seem so distinct. 



Genesis of the Horse. — In conclusion, it will be interesting and in- 

 structive to run out one of these branches and show in more detail the 

 genesis of one of the extreme forms. Eor this purpose we select the 

 Horse, because it has been somewhat accurately traced by Huxley and 

 by Marsh. About thirty-five or forty species of this family, ranging 

 from the earliest Eocene to the Quaternary, are known in the United 

 States. The steps of evolution may therefore be clearly traced. 



In the lower part of the Eocene basin ( Coryphodon teds) of Green 

 River is found the earliest known animal in the direct line of descent of 

 the horse family, viz., the recently-described Eohippus of Marsh. This 

 animal had three toes on the hind-foot and four perfect, serviceable toes 

 on the fore-foot ; but, in addition, on the fore-foot an imperfect fifth 



