358 THE ANATOMY OF VEETEBEATED ANIMALS. 



The existing Equidce are natm-aliy restricted to Europe, 

 Asia, and Africa ; and are distinguished into the Horses, 

 which have horny patches on the inner sides of both pairs of 

 limbs^above the wi-ist in the fore limb and on the inner 

 side of the metatarsus in the hind limb ; and the Asses, 

 which possess such callosities only on the fore limbs. 



Fossil remains of Equidce are abundant in the later ter- 

 tiary deposits of Europe, Asia, and the Americas ; but the 

 group is not known to be represented eai'lier than the 

 miocene, or later eocene, epoch. 



The EquiddB are among the very few groups of Mam- 

 malia, the geological history of which is sufficiently well 

 known, to prove that the existing forms have resulted from 

 the gradual modification of very different ancestral types. 

 The skeleton of the older pliocene and newer miocene 

 Sipparion very closely resembles that of an Ass, or a mo- 

 derate-sized Horse. There is a curious depression on the 

 face in front of the orbit, somewhat like that which lodges 

 the " larmier " of a stag (traces of which are observable in 

 some of the older species of Equus) ; othei'wise the cranium 

 is altogether like that of a Horse. Again, the shaft of the 

 ulna is very slender, but it is larger than in the Horse, and 

 is distinctly traceable throughout its whole length although 

 firmly ankylosed with the radius. The distal end of the 

 fibula is so completely ankylosed with the tibia, that, as in 

 the Horse, it is difficult to discern any trace of the pri- 

 mitive separation of the bones. But, as has been already 

 mentioned, each limb possesses three comj^lete toes — one 

 strong, median, and provided with a large hoof, while the 

 two lateral toes are so small that they do not extend beyond 

 the fetlock joint. In the fore limb, rudiments of the first 

 and fifth toes have been found. 



The teeth are exceedingly like those of the Horse, but the 

 crowns of the molars are shorter; and, in the upper jaw, 

 that which, in the true Horses, is a large fold of the inner 

 face of the tooth becomes a detached piUar. The smaller 

 plications of the enamel are also more numerous, close-set, 

 and complicated. On the outer face of the lower milk 



