ITS ANCESTORS AND RELATIONS 35 



as in the direct line of ancestry of the modern ~Eqividce^ 

 which the true Palceotherium probably was not. One 

 of the most striking characters by which it differs from 

 Hyracotherium is the complete loss of the fifth digit of 

 the fore-foot, all the extremities being alike in possess- 

 ing only the three middle toes (second, third, and fourth 

 of the typical condition), all reaching to the ground, but 

 with the central one (the third) longer than the others 

 (see fig. 4). The two bones of the fore-arm (radius and 

 ulna) and the two of the leg (tibia and fibula) were still 

 quite distinct. The pattern of the grinding surface of 

 the molar teeth (see fig. 3, C, p. 30) had undergone some 

 further modifications from that of Hyracotherium, which 

 will be alluded to later on when describing the dentition 

 of the horse. The Anchitherium was succeeded in the 

 Pliocene period, in America, Europe, and Asia, by 

 animals which have been named Hipparion, Hippo- 

 therium, Protohvppus, and Pliohippus, of which there were 

 many kinds, differing slightly in form and proportions, 

 and in the characters of the enamel foldings of the molar 

 teeth, but resembling each other in the structure of the 

 feet. The lateral toes (second and fourth), though con- 

 taining the full number of bones, were much reduced in 

 size, and did not reach to the ground (see fig. 5), but were 

 suspended to the outside of, and rather behind, the large 

 middle one, like the rudimentary outer toes of the deer or 

 the short first digit (' dew-claw ') of the dog. Well- 



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