PERISSODACTYLA. 



313 



recent species of the Equities the number is reduced to six, but there is a small 

 tooth in front of the first prasmolar which soon falls out (Wolfszahn, Bojanus). 

 Fossil forms first appear in the eocene (Orohippus still with a rudimentary 

 fifth digit, as well as the three other digits which rested on the ground, and 

 Anchitherium}. They persisted in the miocene and pliocene (Hipparion) and 

 then pass into the diluvial genus Equus, to which the domestic horses of the 

 present day belong. Ancfutlieriumliumasli Gerv. Feet with three digits, middle 



FIG. 690. Pedal skeleton of different genera of Equida (after Marsh), a, Foot of Orokippv 

 (Eocene); b, Foot of Anchitherium (Lower Miocene ; c, Foot of Hipparion (Pliocene); d, 

 Foot of the recent genus Equw}. 



digit large, remains of fifth metacarpal on the anterior limbs. The grinders are 



7 P 4*4 3'3 ~\ 



7. &' m ' 4^4* m ' 3^3- Hipparlon gracile Kp., miocene. Of the seven grinders, 



the anterior is a simple prism with a semilunar transverse section ; it is lost 

 wit!h the milk dentition. Equus caballus. Foot composed of one digit, with 

 remains of metatarsals (metacarpals) of second and fourth digits (splint 



Fia. 691. Skull of Equus caballus. 



o.o 



bones); grinders are P- m. __. 



u'O 



r-x, with remains of an anterior seventh 

 o'o 



grinder m the milk dentition. This genus is only known in the domestic 

 state, bu t is probably descended from one or several of the species of horses 

 which lived in the diluvial period. Asinus tceniopus Heugl., the wild ass of 

 South-East Asia, the ancestral form of the domestic ass (E. asinus)\ 

 A. hemionvs Pall., Dziguetai ; A. onager Pall., Kulan, Mongolia. The African 



