SKULLS OF SOME ANCESTORS OF THE HORSE 



1. Protoidliippus venticolus. 



2. Mesohippus Bairdi. 



3. Hipparion gracilis. 



4. Onohippidium Munizi (an extinct South American horse). 



5. Arab Horse. 



A is a cavity for a face gland, and is very marked in 

 Hipparion (3) and Onohippidium (4), while a vestige 

 of it remains in the Arab horse (.5). It is absent 

 from some breeds of living horses. 



B is the remarkably long nasal slit which is a very 

 noticeable feature of Onohippidium (4). 



