HISTORY OF THE fLITOPTERNA 505 



skull was light, slender and pointed ; the nasals were shortened, 

 though less than in \Diadiaphorus ; the sagittal crest was 

 shorter than in the latter and the occiput was far narrower. 

 The neck was short, the body of moderate length and the tail 

 short. The Umbs and especially the feet were proportionately 

 more elongate and slender than in any other known genus of 

 the family, giving quite a stilted appearance to the skeleton. 

 The fore-arm bones were not coossified, but the ulna was much 

 more reduced than in any of the other genera of the family, 

 and the same is true of the fibula, which, though very slender, 



Fig. 251. — SkiiU of \Thoatheriv,m, Santa Crviz. Princeton University Museum. 



showed no tendency to unite with the tibia. The limb-bones, 

 especially the femur, had a decided resemblance to those of 

 \Mesohi'p'pus, the lower Oligocene tridactyl horse of North 

 America, with the smaller species of which, ^M. bairdi, ^Thoa- 

 therium agreed well in size. Most remarkable of all were the 

 feet, which were more strictly monodactyl than those of any other 

 known mammal. The single functional digit, the third, had 

 on each side of its upper end a very small, scale-like nodule of 

 bone, the last vestiges of the lateral digits, corresponding to 

 the immensely larger splints of the horse. Despite the un- 

 rivalled c6mpleteness of digital reduction which ^Thoatherium 

 displayed, the mode of reduction was inadaptive and the 

 rudimentary metapodials retained the same carpal and tarsal 



