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; Ayracotherium and 
Systemodon had only four functional digits in the manus ; Amchithertam 
from the Miocene, an animal about the size of a sheep, had three digits, 
or three and a rudiment; Azppotherium 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 (Agus caballus), apparently 
originating in Asia, domesticated in prehistoric times, artificially selected 
. 411.—Feet of horse and its predecessors. — 
From Neumayr. : 
x, Palzotherium ; 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 (2. prsevalskez) ; the donkey (Z. aszuus) of 
African origin ; the wild asses of Africa and Asia; the striped African 
species—the zebras and the (exterminated) quagga. 
Family Rhinocerotide.—There is now but one genus, RAznoceres, 
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 xs huge warts from the snout 
and forehead. The dentition is very variable, but the back 
teeth re are almost uniform; there are no upper canines, but 
y 
