4 6 



THE DESEADO FORMATION OF PATAGONIA 



convex external condyle is much narrower anteroposte- 

 rior^' than the larger and slightly concave internal condyle. 

 The low spine is bifid. A cnemidial 

 (rest extends to the middle of the bone. 

 On the distal end, the broad and shal- 

 low external articular facet is separated 

 from the narrow and deeper internal 

 facet by a low intercondylar ridge. 

 The fibula is fused to the tibia at 

 the upper end, but is free below, being 

 approximated to the tibia along a rugose 

 surface nearly an inch long. This bone 

 is rather slender and strongly bowed 

 outward. Distally, there is a large 

 facet for the outside of the astragulus, 

 the back part of which rests on the 

 calcaneum. This is peculiarly devel- 

 oped so that the articulation represents 

 what is two separate facets, the one for 

 the outside of the astragulus the other 

 for the calcaneum. Here, however, they 

 are blended. 



While in general the tarsus is similar 

 to that of Theosodon, there are some 

 marked contrasts. The astragulus has 

 an asymetrical tro- 

 chlea with a shallow 

 groove, the external condyle being higher 

 and narrower than the internal. The 

 head is depressed in the dorso-plantar 



• . . 111 Fis- 20. Astragalus 



plane, is carried on a moderately long plantar side— a, exter- 



. . . nal facet; b, sustentac- 



neck, and has a broad convex facet lor mar facet— 1/2 natural 



size. 



the navicular on which alone it articu- 

 lates. On the plantar side, the ectal facet is broadly oval 

 and slightly concave, differing from that of Theosodon in 

 having no sulcus dividing it into lobes. The broad sus- 

 tentacula!- facet is slightly convex, and widely separated 



\<>. Right foot 



in outline from 

 the left foot— 1/2 natural 

 size. 



Fi 

 phalange 



