EVOLUTION OF THE HORSE. 523 



Gaudry (i.) The first, second, and fifth metatarsals and first cunei- 

 form bone are carried backward; (2.) the posterior proximal part of 

 the third metatarsal is enlarged to sustain the cuneiform ; (3.) the first 

 and second cuneiform elements of the tarsus, the second and fifth 

 metatarsals, and the trapezius become very small; (4.) various ele- 

 ments of the metacarpus and metatarsus are blended. 



The ancestry of the Horse offers some difficulties which impose 

 caution, for one line of descent for the Old World horses is suggested 

 by European specimens, while Professor Marsh has developed another 

 line of descent for the horses of America. In Europe the horse is 

 traced from Paloplotlierium by way of Pachynolyphus to Anchitherium, 

 which is allied by its teeth to Palceotherium, though its limbs ap- 

 proximate to the more slender forms of Paloplothvrium and Hipparion, 

 which last is little more than a horse with two small supplemental 

 digits, one on each side of the hoof. Monstrosities of the living horse 

 sometimes have a supplemental digit developed with small hoof, such 

 as is found in the Miocene Hipparion : Professor Marsh has figured 

 the eight-hoofed Cuban horse, in which there is one such supplemental 

 digit on each foot. Such a horse as Equus stenonis of the Pliocene, in 

 which the teeth closely approximate to Hipparion, is regarded as the 

 immediate ancestor of the Old World horse. In the characters of the 

 extremities of the limbs, Acerotherium of the Middle Miocene diverges 

 considerably from the type of the existing horse, for it has three large 

 nearly equal digits, with a small one on the outside. PalceotJterium 

 makes a nearer approximation, for it has three digits, of which the 

 middle one is slightly the largest. In Paloplotherium the lateral 

 digits and metapodial bones are much reduced in size ; while in 

 Hipparion the reduction is carried a stage further, and the middle 

 digit is proportionately enlarged. 



The American ancestors of the horse are of similar type to those 

 which are known in Europe. Eohippw had four well-developed toes, 

 and the rudiment of another on each forefoot, and three toes behind. 

 It was no larger than a fox. It is found near the base of the Eocene, in 

 beds which yield Coryphodon. A little higher up in the Eocene is the 

 Orohippus, which was of similar size, but had four toes in front and 

 three behind, the middle one being the stouter. At the top of the 

 Eocene is a third equine animal, termed by Professor Marsh Epihippus. 

 It differed from its predecessor more in its teeth than in the digits. 

 At the base of the Miocene is the Mesohippus, which was about as 

 large as a sheep. It had three well-developed toes, and the splint of 

 another on each forefoot, and three toes behind. In newer beds the 

 animal is termed Mioliippus. It has the splint bone of the outer 

 digit reduced to a short rudiment, and the middle digit is becoming 

 relatively stout. It may be compared with the Ancliitlierium. In 

 the Pliocene beds is a three-toed horse as large as a donkey, termed 

 Protohippus, which may be compared with Hipparion, and, like its 

 European representative, differs from the older horse ancestors in the 

 wavy complexity of the tooth enamel. Newer sti'l is a near ally of the 

 modern horse termed Pliohippus, which has lost the lateral digits, and 



