138 PATAGONIAN EXPEDITIONS: FALyEONTOLOGY. 



On the fibular side of the head is a deep sulcus for the tendon of the 

 extensor longus digitorum. The proximal facet for the fibula is small and 

 is confined to the postero-external angle of the tibial head. 



The shaft is heavy and nearly straight, with trihedral proximal portion, 

 gradually contracting below and expanding again near the distal end, 

 where it becomes irregularly oval in cross-section. The distal end is 

 narrower and thicker than in MacrmicJienia and is divided into a broader 

 external, and narrow and deeper internal portion, by a low intercondylar 

 ridge. Anteriorly, this intercondylar ridge is produced into a short, blunt 

 tongue, while the posterior tongue is much more prominent and larger 

 in every dimension. In some individuals the astragalar surface is inter- 

 rupted by a small, shallow sulcus, which is chiefly on the external condyle 

 and the intercondylar ridge. (See fig. 5«.) The internal malleolus is 

 singularly small and confined to the dorsal moiety of the internal side ; 

 in Macrauchenia this process is still further reduced and has, in fact, 

 almost disappeared. The distal fibular facet is large and oblique. 



Of the fibula (PL XX, figs. 6, 6a) only the distal half is preserved in 

 the collections. That the proximal end was small, is shown by the facet 

 for it on the head of the tibia. The shaft is much more reduced and 

 slender than that of the ulna and its middle portion is quite thin and 

 laterally compressed, though retaining considerable antero-posterior thick- 

 ness. Distally, the shaft increases steadily in thickness. The distal end 

 which is stout and forms a heavy external malleolus, bears three facets ; 

 on the inner side is an inclined surface, which is overlapped by the tibia, 

 and this meets at an obtuse angle the surface for the outer condyle of the 

 astragalus. For the calcaneum is a large distal facet, consisting of an 

 anterior convexity and posterior concavity, though there is no interruption 

 in the continuity of the articular surface. On the outer side of the distal 

 end is a conspicuous groove for the peroneal tendon. 



As in the case of the manus, the pes (PI. XX, fig. 7) is very similar to 

 that of Macrauchenia, with only a few minor differences. The tarsus 

 (PI. XX, figs. 7-9) which is relatively high and narrow, maintains in all 

 respects the character of the Litopterna and differs in points of fundamen- 

 tal significance from that of the Perissodactyla, despite the mesaxonic 

 symmetry of the foot. 



The astralagus has quite a perissodactyl appearance, which, however, 

 is entirely superficial in character. The trochlea is deeply grooved and 



