THE LAEGE TUSHES OF OROHIPPUS. 97 



teeth, rapidly increased in size, and finally became the 

 largest of tlie series. The grinding teeth had at first 

 very siiort crowns, without cement, and were inserted 

 by distinct roots. In Pliocene species the molars be- 

 came longer, and were more or less coated with cement. 

 The modern horse has very long grinders, without 

 true roots, which are covered with a thick external 

 layer of cement. The large canines of Orohippus be- 

 came gradually reduced in the later genera, and the 

 characteristic *mark' upon the incisors is found only 

 in the later forms. It is an interesting fact that the 

 peculiarly equine features acquired by Orohippus are 

 retained persistently throughout the entire series of 

 succeeding forms." * 



* "The ancient Orohippus liad all four digits of the fore-feet 

 well developed. In Mesohippus, of the next period, the fifth 

 toe is only represented by a rudiment, and the limb is supported 

 by the second, third, and fourth, the middle one being the 

 largest. Hipparion of the Later Tertiary still has three digits, 

 but the third is much stouter, and the outer toes have ceased to 

 be of use, as they do not touch the ground. In Equus the lat- 

 eral hoofs are gone, and the digits themselves are represented 

 only by the rudimentary splint-bones. The middle or third 

 digit supports the Umb, and its size has increased accordingly. 

 The corresponding changes in the posterior limb of these genera 

 are very similar but not so striking, as the oldest type (Orohip- 

 pus) had but three toes behind. The earlier ancestor of the 

 gfroup, perhaps in the lowest Eocene, probably had four on this 

 foot and five in front. Such a predecessor is as clearly indicated 

 by the feet of Orohippus as the latter is by its Miocene relative. 

 A still older ancestor, possibly in the Cretaceous, doubtless had 

 five toes on each foot, the typical number in mammals. This 

 reduction in the number of toes may perhaps have been due to 

 elevation of the region inhabited, which gradually led the ani- 

 mals to live on higlier ground, instead of the soft lowlands, 

 where a many-toed foot would be most useful." — Prof. 0. (7. 

 Marsh. 



