Evolution of the Horses 191 



more than a thread of bone. The skull has a longer face and a nearly 

 enclosed orbit, and the brain-case is fuller and more capacious, the 

 internal cast of Avhich shows that the brain was richly convoluted. 

 The teeth are still very short-crowned, but the upper incisors plainly 

 show the beginning of the " mark " ; the premolars have assumed the 

 molar form, and the upper molars, though plainly derived from those 

 of Eohippus, have made a long stride toward the horse pattern, in 

 that the separate cusps have united to form a continuous outer wall 

 and two transverse crests. 



In the lower Miocene the interesting genus Desmatipjms shows 

 a further advance in the development of the teeth, which are beginning 

 to assume the long-crowned shape, delaying the formation of roots ; 

 a thin layer of cement covers the crowns, and the transverse crests 

 of the upper grinding teeth display an incipient degree of their 

 modern complexity. This tooth-pattern is strictly intermediate 

 between the recent type and the ancient type seen in Mesohippus 

 and its predecessors. The upper Miocene genera, Protohqjpiis and 

 Hipparimi are, to all intents and purposes, modern in character, but 

 their smaller size, tridactyl feet and somewhat shorter-crowned teeth 

 are reminiscences of their ancestry. 



From time to time, when a land-connection between North 

 America and Eurasia was established, some of the successive equine 

 genera migrated to the Old World, but they do not seem to have 

 gained a permanent footing there until the end of the Miocene or 

 beginning of the Pliocene, eventually diversifying into the horses, 

 asses, and zebras of Africa, Asia and Europe. At about the same 

 period, the family extended its range to South America and there 

 gave rise to a number of species and genera, some of them extremely 

 peculiar. For some unknown reason, all the horse tribe had become 

 extinct in the western hemisphere before the European discovery, but 

 not until after the native race of man had peopled the continents. 



In addition to the main stem of equine descent, briefly considered 

 in the foregoing paragraphs, several side-branches were given oif at 

 successive levels of the stem. Most of these branches were short- 

 lived, but some of them flourished for a considerable period and 

 ramified into many species. 



Apparently related to the horses and derived from the same 

 root-stock is the family of the Palaeothcrcs, confined to the Eocene 

 and Oligoccne of lOurope, dying out without descendants. In tlie 

 earlier attenqjtH to work out the history of the horses, as in the 

 famous essay of Kowalevsky\ the Palaeotheres were placed in the 

 direct line, because the ninnhcr of adecjuately known Eocene mam- 



* "Sur Y Anchitherium aurc/ianen«e Cuv. et Bur I'liistoirp paliontologiquedeeChevaux," 

 M(m. de VAcad. Imp. de$ Sc. de St Piterthourg, xx. no. 6, 1873. 



