PERISSOD ACTYLA. 339 



Paloiothcridce. Both the hind and fore feet (fig. 637, b) are 

 three-toed, all the toes being sufficiently developed to touch 

 the ground, but the central toe (third digit) is much the 

 largest. There is likewise no splint-bone representing the 

 fifth digit in the hand ; while the genus further differs from 

 Mioliippus in having a large antorbital fossa. 



Another important European type of Equidm is Hipparion, 

 which seems to have abounded in Southern Europe during 

 the later Miocene and Pliocene periods. In this genus, the 

 skeleton is like that of the Horse in its general conformation ; 

 but the feet, though functionally single-toed, are anatomically 

 three-toed. The central toe (third digit) is now the only toe 

 which touches the ground (fig. 637, c) ; while the second and 

 fourth digits, though visible externally and furnished with 

 small hoofs, are so much reduced in size as to have taken no 

 part in supporting the weight of the body. The teeth are 

 very horse-like, but an antorbital fossa was present. 



In the Lower Pliocene of North America, the place of 

 Hi-pparion is taken by Frotohippus, some of the species of 

 which equalled the Ass in size, while the structure of the 

 feet resembled that of Hipparion. 



Of the other Pliocene horses of America, the only one of 

 importance is Plioliippus, in which the foot has the same 

 structure as in Equus (fig. 637, d). That is to say, the feet 

 have only a single functional toe each, and the second and 

 fourth toes are only rej^resented by rudimentary " splint- 

 bones," concealed beneath the skin. Though agreeing with 

 Equvs in the structure of its feet, Fliohippus differs from this 

 genus in possessing a large antorbital fossa, and in having 

 an additional permanent upper pra3molar, the dental formula 

 being; — 



.3—3 1—1 4—4 3—3 



^ ; c ; pm^ ; on = 42 



3_3 i_i 3—3 3—3 



Lastly, the genus Eqmis itself seems to have made its first 

 appearance towards the close of the Miocene or the com- 

 mencement of the Pliocene period. In the Old World the 

 first appearance of true Horses seems to be in the Equus 



