40 THE DESEADO FORMATION OF PATAGONIA 
convex external condyle is much narrower anteroposte- 
riorly than the larger and slightly concave internal condyle. 
The low spine is bifid. A cnemidial 
crest 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 
caleaneum. 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 
idisis to that of Theosodon, there are some 
Fig. 19. Right foot, the marked contrasts. The astragulus has 
phalanges in outline from 
the left foot—1/2 natural 
Seas 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 
. 5 Fig. 20. Astragulus 
plane, is carried on a moderately long plantar side—a, exter- 
nal facet; 6, sustentac- 
neck, and has a broad convex facet for ular facet—r/2 natural 
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- 
tentacular facet is slightly convex, and widely separated 
