298 VERTEBRATE ANIMALS. 



America, Sumatra, and Malacca. The third and last family of 

 the Perissodactyla is that of the Equidm, comprising the 

 Horse, Ass, Zebra, and Quagga. In this family the toes are 

 reduced to one to each foot, enclosed in a single broad hoof, 



Fig. 148. — Head of Two-homed Ehinoceros (72. Mcorrm). 



without any supplementary hoofs. There is a continuous 

 series of incisor teeth in both jaws, and in the males canines 

 are present. The dental formula is : 



. 3—3 1—1 , , 3—3 3—3 .. 



* 3=3' 'i=i ^°' ^°''*^) ; .?"^ 3=3; '"s^S = ^° 



All the varieties of Horses appear to be descended from 

 the single species Equus cdballus, which seems to have been 

 primitively a native of Central Asia. When the American 

 Continent was discovered it certainly possessed no living horse, 

 but the horse has now become completely naturalized there, 

 and we know that America formerly possessed about twenty 

 species of horses, all of which are now extinct. In the genus 

 Asinus are the Asses, Zebras, and Quaggas. The Wild Ass 

 is a native of Asia, and the domestic Ass is probably descended 

 from it. The Zebras and Quaggas are exclusively African, and 

 are distinguished by their beautifully-striped and banded 

 bodies. 



The Artiodaotyles or Even-toed ZTngulates are divided into 

 two groups : 



1. Omnivora, as the Pig and Hippopotamus. 



3. Muminantia, which chew the cud, such as Oxen, Deer, 

 Camels, etc. 



Of the Omnivorous forms the Hippopotamus or River- 



