PERISSODACTYLA 



491 



considered a relatively unsuccessful group. Its members are 

 herbivorous, and the legs are lengthened for speed so that the 

 animal walks not merely on its toes but on its nails, so that it is 

 called unguligrade. The emphasis in the limbs is on the middle 

 digit, so that when, as is usual, the digits are reduced, their number 

 tends to be odd. In the modern horse {Equus, Figs. 378-79), this 

 reduction has continued until only a greatly enlarged third toe 

 is left, with small functionless splint bones representing the second 

 and fourth metacarpals and metatarsals ; there are other modi- 

 fications for a rapid fore-and-aft movement of the limbs, such as 



Fig. 380. — A vertical section 

 of an incisor tooth of a 

 liorse. — From Tfieobald, 

 after Chauveau. 



C, Cement ; E, enamel ; /, dentine. 



Fig. 381. — A transverse 

 section of an upper 

 molar tooth of a horse. 

 — From Theobald, af- 

 ter Chauveau. 



A, External cement ; B, ex- 

 ternal enamel; C, dentine; 

 D, internal enamel ; E, in- 

 ternal cement. 



pulley-like joints, reduction of the ulna and fibula to mere 

 vestiges, and absence of the clavicle. The incisors (Fig. 380) slope 

 forward and are used for cropping, the canine is reduced, and 

 generally absent from the mare, and the cheek teeth (Fig. 381) 

 have a complicated pattern produced by wear. The stomach is 

 simple, but bacterial digestion of cellulose takes place in the 

 caecum and large intestine. 



The orders Hyracoidea (the conies of the Old Testament, but 

 not those of Ehzabethan writers and the law, which were rabbits), 

 Proboscidea (elephants), and Sirenia (sea cows) have no living 

 British representatives. The conies superficially resemble rabbits, 

 from which they can be distinguished by the triangular section 

 of the upper incisors. The chief features of an el-phant are its large 

 size, its trunk, which is an elongated and prehensile nose and 



