686 



EVOLUTION OF THE HORSE. 



a very lucid description of the structural characteristics of 

 this animal, wliich apparently has left no descendants. 

 The bones of a horse's fore leg, below the fore-arm, are 

 shown in Fig. 640. 



One of the most remote ungulate ancestors of the horse 

 is Phenacodiis priniccvus (Fig. 641), which " is antecedent 

 to all the horse series, the hog, the rhinoceros, and all the 

 other series of hoofed animals " (Cope). Phenacodus, 

 which belongs to the early Eocene period, had five digits 



Fig. fa4i. — I'lienacciclus primtevus (r'j actual size)'. 



on each limb, and three phalanges on each digit, which 

 is a more complete arrangement of the hands and feet 

 than is possessed by any normal animal. In our own 

 case, the thumb and big toe have only two phalanges, 

 and in the little toe, two of the three phalanges are some- 

 times united. In Phenacodus, the ulna and fibula were 

 entire, and separated from, respectively, the radius and 

 tibia ; and there was no interlocking of the carpal (wrist) 

 bones. Phenacodus had what is regarded as a typical set 

 of teeth, namely, 22 in each jaw (44 teeth in all). The 

 incisors were small and had sharp edges, and the canines 



