58 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
in cross-section proximally, but becomes oval in section toward the distal 
end, where it is expanded transversely. Internally and proximally, it is 
broad and almost a plane surface, while externally it is concave, the 
prominent cnemial crest forming the dividing line between the two sur- 
faces. Distally, the articular surface closely resembles that of Protypo- 
therium. The trochlea is evenly divided by a broad median keel. The 
internal malleolus is robust, with a large, slightly concave surface for the 
internal astragalar crest. Distally, the internal malleolus terminates in a 
hook-shaped process directed outward and downward (PI. VIII, fig. 5). 
The fibula is a slender element, firmly coossified with the tibia proxi- 
mally and articulating with it distally by suture. The shaft is irregularly 
oval in cross-section, except at the ends, where it is transversely com- 
pressed proximally and expanded, with triangular outline, distally. The 
latter extremity supports a crescentic, plane surface for the outer astraga- 
lar crest and a second crescentic surface antero-posteriorly sigmoid in sec- 
tion for the calcaneum. The peroneal groove on the posterior surface of 
the distal end is quite conspicuous. 
The pes is tetradactyl, with no trace of the hallux. Ameghino (1891 b, 
p. 393, fig. 95, reproduced as text fig. 10, B ) refers to Interatherium 
(. Icochilus ) robustum a large hind foot, with strongly opposable hallux. 
An examination of the numerous complete hind feet in the Princeton and 
American Museum collections shows that this foot, like the fore foot men- 
tioned on page 56 does not pertain to Interatherium or to any of the 
Santa Cruz Typotheria. The astragalus of Interatherium closely resem- 
bles that of Protypotherium , differing in the slightly shallower trochlea, 
proportionately shorter neck and the lesser degree of median constriction 
in the calcaneal facet. The calcaneum also is strikingly like that of Pro- 
typotherium , differing in the reniform shape of the astragalar facet and its 
sharper differentiation from the fibular facet than in the latter genus. 
With the exception of minor differences in the shape and proportions of 
the facets, the tarsus is closely similar to that of Protypotherium and need 
not be described in detail. The third and fourth metatarsals are equal in 
length, as are also the second and fifth, but shorter than the other two. 
All the metatarsals interlock proximally with each other and with the tar- 
sus. Well developed keels are present on the distal plantar margin. 
The phalanges can be distinguished from those of the manus only by their 
greater size. They are slightly expanded transversely both proximally and 
