ix.] PALAEONTOLOGY AND EVOLUTION. 195 



verted into Equus is one of specialization, or of more and 

 more complete deviation from what might be called the 

 average form of an ungulate mammal. In the Horses, 

 the reduction of some parts of the limbs, together with 

 the special modification of those which are left, is carried 

 to a greater extent tharn in any other hoofed mammals. 

 The reduction is less and the specialization is less in 

 the Hipparion, and still less in the Anchitherium ; but 

 yet, as compared with other mammals, the reduction 

 and specialization of parts in the Anchitherium remain 

 great. 



Is it not probable then, that, just as in the Miocene 

 epoch, we find an ancestral equine form less modified 

 than Equus, so, if we go back to the Eocene epoch, we 

 shall find some quadruped related to the Anchitherium, 

 as Hipparion is related to Equus, and consequently 

 departing less from the average form \ 



I think that this desideratum is very nearly, if not 

 quite, supplied by Plagiol&phus, remains of which occur 

 abundantly in some parts of the Upper and Middle 

 Eocene formations. The patterns of the grinding-teeth 

 of Plagiolophus are similar to those of Anchitherium, and 

 their crowns are as thinly covered with cement ; but the 

 grinders diminish in size forwards, and the last lower 

 molar has a large hind lobe, convex outwards and concave 

 inwards, as in Palwotherium. The ulna is complete and 

 much larger than in any of the Equidce, while it is more 

 slender than in most of the true Palceotheria ; it is 

 fixedly united, but not ankylosed, with the radius. 

 There are three toes in the fore limb, the outer ones 

 being slender, but less attenuated than in the Equidce. 

 The femur is more like that of the Palceotheria than that 

 of the horse, and has only a small depression above its 

 outer condyle in the place of the great fossa which is so 

 obvious in the Equidce. The fibula is distinct, but very 



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