96 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



very thin and the expansion of the ends is chiefly in the antero-posterior 

 direction ; the sulcus for the peroneal tendon is even deeper than in Pro- 

 teyothermm and its recurved anterior border ends in a larger and more 

 rugose tubercle. Anteriorly, the distal end extends somewhat beneath 

 the tibia, a ruminant rather than a perissodactyl character ; the facet for 

 the calcancum is elongate antero-posteriorly, but very narrow, and that 

 for the astragalus is quite large and C-shaped and is principally confined to 

 the anterior portion of the distal end. 



Of particular interest is the pes (PI. XIII, figs. 13, 13(7) of Thoatherhtm, 

 as showing the extreme of inadaptive modification of the monodactyl foot, 

 just as the horses display the extreme of adaptive monodactylism. Further, 

 these fossils present us with an extraordinary case of convergent develop- 

 ment, since no one, after a study of them, will be inclined to bring the 

 Proterotheriidae into any close relations with the horses. 



In the tarsus there is relatively little change from Proterotherium. The 

 proportions of the astragalus as to length and width are the same as in 

 the latter and form a strong contrast to the short, broad astragalus of the 

 horse : the condyles have a slight obliquity, which varies in different indi- 

 viduals, and have narrower borders than in Proterotherium, while the 

 articular surface is not reflected so far over upon the tibial side ; the pit 

 on the dorsal side of the neck for the anterior distal tongue of the tibia is 

 shallower than in the latter and its distal border is less raised. The two 

 parts of the ectal calcaneal facet meet at a more acute angle and the surface 

 is much more deeply notched by an incision from the fibular side. The 

 neck is shorter and the navicular facet is reflected farther upon the dorsal 

 face along the fibular border ; the sustentacular facet narrows more prox- 

 imally, somewhat as in DiadiapJiorus. 



The calcaneum differs from that of Proterotherium in a number of par- 

 ticulars : the tuber, which is of the same relative length and similar form, 

 is rather more compressed and slender, with less thickened and club-like 

 free end, which has a straighter proximal border, owing to the less promi- 

 nent projection of the ventral portion : the fibular facet is somewhat 

 broader and less inclined toward the tibial side : the sustentaculum has a 

 more abruptly truncated inner side and the distal end of its facet is carried 

 up farther upon the body of the calcaneum : the distal end is considerably 

 shallower planto-dorsally. 



The navicular is like that of Proterotherium, but the plantar border of 



