PROTOHIPPUS. 691 



the tables of the molars were not filled with cement. 

 The term, Miohippus, means "Horse of the Miocene." 



Protohippus (Fig. 643). — " The genus Protohippus 

 of the Lower Pliocene is yet more equine, and some of 

 its species equalled the ass in size. There were still 

 three toes [2nd, 3rd and 4th] on each foot, but only the 

 middle one, corresponding to the single toe of the horse, 

 comes to the ground. This genus resembles most nearly 

 the Hipparion of Europe " {Marsh). Respecting Professor 

 Marsh's remark about the 2nd and 4th toes not coming 

 to the ground, we have the fact, already alluded to (p. 

 320), that some horses, especially high-caste Arabs, have 

 such a large amount of " play " in the fetlock and 

 pastern joints of the fore-legs, that during the fast gallop, 

 the fetlock pad frequently comes down on the ground, 

 and is liable to get bruised. As this " play " (exten- 

 sion) of the joints of the foot must have been much 

 greater in the Protohippus than in our horses, and as 

 the 2nd and 4th digits of the fore legs of this equine 

 fossil were nearly on a line with the coronet of the 3rd 

 digit ; the decadent digits of Protohippus must have 

 come to the ground at fast paces, if not in slow move- 

 ment, especially on soft soil, and when going down hill. 



Protohippus, which lived chiefly on plains, seems to 

 have been evolved in North America, from which it 

 extended southwards into South America, and cast- 

 wards into Asia. As the last vestige of the 5th digit 

 vanished ; the inner incisor became cupped — like the 

 middle and outer incisors — and developed a '' mark " ; 

 and there was abundant cement. Though the 2nd, 3rd 

 and 4th premolars and all the molars had longer crowns 

 than in the less specialised Miocene forms, the ist pre- 

 molar was of considerable size. The orbit was complete 

 in Protohippus, and the skull closely resembled that of 

 living horses. 



44^ 



