14 



THE AGE OF MAMMALS 



tral digits (Artiodactyla). This difference was expressed in other words 

 by Alarsh ^ by the terms Mesaxonia and Paraxonia respectively. 



The pose or angle of the foot as a whole, including the wrist and ankle 

 joints (carpals and tarsals), the intermediate bones (metacarpals and 

 metatarsals), and the terminal joints (phalanges and ungues), began to 



Wrjst 



Fig. 5. — • Evolution from pentadactyl to monodactyl condition as illustrated in the foot of 

 the horse (above) ; comparison with the human hand (below). A. Pentadactyl, hypothetical 

 ancestral stage. B. Tetradactyl, Protorohippus stage. C. Tetradactyl-tridactyl, Epihippus 

 stage (three toes resting on the ground, one raised slightly above). D. Tridactyl, Mesohippus 

 E. Monodactyl, Equus stage. 



attract increasing attention. In primitive forms it was seen that the entire 

 hand and foot is stretched along the ground, as in the bear and in the foot 

 of man, while in the opposite extreme the foot rests on the nail or on the 

 very extremity of the last phalanx, as in the hoof of the horse. Thus in the 

 gradus, or step, a series of important intermediate stages are observed, as 

 follows : 



Plantigradism, where the entire sole of the hand or foot rests upon the 



ground, as in the hind foot of the bear. 

 Semi-plantigradism, where half the sole rests upon the ground, as in the 



fore foot of the bear. 



' Marsh, O. C, Dinocerata. A Monograph of an Extinct Order of Gigantic Mammals. 

 U. S. Geol. Surv., Vol. X. Washington, 1884, p. 186. 



