ORDERS OF MAMMALIA. 333 



only Ciree toes on the hind-legs, so that they are really odd-toed. 

 The nose forms a short movable proboscis, used in stripping off the 

 leaves of trees. They are large, clumsy, pig-like animals, which 

 inhabit South America, Sumatra, Borneo, and Malacca. 



The third and last family of the living Perissodactyle Ungulates 

 is that of the Equidce, comprising the Horses, Asses, Zebras, and 

 Quaggas. In this family, so far as existing types are, concerned, the 

 foot carries only a single toe (the third toe), which is furnished with 

 a broad hoof (fig. 22.3). Upon the end of this toe the animal walks, 

 and the Eqvddce are, therefore, often spoken of as " Solipedes." The 

 second and fourth toes are present in a rudimentary form as little 

 bony splints (the so-called " splint-bones "), hidden under the skin 

 (fig. 22.3) ; but in certain extinct types these lateral toes are more or 

 less developed, and carried hoofs, the foot thtis becoming three-toed. 



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 ,„ 



I ; c - - (or none) ; pm — : m = 40. 



3-3 1-1 ^ ' ' 3—3 3—3 



All the domestic varieties of Horses appear to l,)e descended from 

 the single species Eqims cahallus, which seems to have been primi- 

 tively a native of Central Asia, in which country another species of 

 Horse has been recently discovered. When the American continent 

 was discovered, it certainly possessed no living horse, but the Horse 

 has now become completely naturalised there, and we know that 

 America formerly possessed about twenty species of Horses, all of 

 which are now extinct. In the genus Asmus are the Asses, Zebras, 

 and Quaggas. The Wild Asses are natives of Asia and North 

 Africa ; but it is not quite certain from what species the domestic 

 Ass is descended. The Zebras and Quaggas are exclusively African, 

 and are distinguished by their beautifully striped and banded bodies. 



Section B. Artiodaciyla. — The Even-toed Ungulates may be divided 

 into the two groups of the Omnivora and the Ruminantia, the for- 

 mer comprising the Hippopotamuses and the Swine, while the latter 

 includes the Oxen, Deer, Camels, &c. 



Of the Omnivorous Artiodactyles, the Hippopotamuses constitute 

 a small group, comprising massive, thick-skinned Ungulates, with 

 four-toed feet. The common Hippopotamus amphibius inhabits all 

 the great rivers of Africa, living upon plants, and swimming and 

 diving with great facility. The molar teeth have flat crowns, of a 

 peculiar double-trefoil pattern ; but the canines are very large, and 

 the lower ones in particular form enormous tusks, with a chisel- 



