762 



MAMMALIA. 



digits, an increased folding of the back teeth, and other differentiations. 

 The Eocene Phenacodus is regarded by some as near the origin of the 

 stock, it had five complete digits on each foot ; Hyracotherium and 

 Systemodon had only four functional digits in the manus ; Anchitherium 

 from the Miocene, an animal about the size of a sheep, had three digits, 

 or three and a rudiment ; Hippotherium and Protohippus from the 

 Pliocene were as large as donkeys, and show a marked diminution of 

 the second and fourth digits ; in the Pliocene also, the modern forms 

 appeared. 



The living species are the horses (Eqims caballus]^ apparently 

 originating in Asia, domesticated in prehistoric times, artificially selected 



FIG. 411. Feet of horse and its predecessors. 



From Neumayr. 

 i, Palaeotherium ; 2, Anchitherium ; 3, Hippotherium ; 4, Equus. 



into many breeds, sometimes reverting to wildness, as in the case of 

 those imported into America and Australia by European settlers ; the 

 wild horse of Central Asia (E. przevalskii) ; the donkey (E. asinus] of 

 African origin ; the wild asses of Africa and Asia ; the striped African 

 species the zebras and the (exterminated) quagga. 



Family Rhinocerotidse. There is now but one genus, Rhinoceros , 

 species of which occur in Africa and in some parts of India and 

 Indo-Malaya. They are large, heavy Ungulates, shy and noc- 

 turnal, fond of wallowing in water or mud, feeding on herbage, 

 shoots, and leaves. The skin is very thick, with scanty hair. 

 One or two median horns grow as huge warts from the snout 

 and forehead. The dentition is very variable, but the back 



teeth 3 are almost uniform ; there are no upper canines, but 



