170 PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 
Santa Cruz genus. The shaft has a similar, laterally compressed shape, 
though it has no distal interosseous crest, but the distal expansion is pro- 
portionately much greater. The division of the astragalar surface is even 
more faintly marked, the intercondylar ridge lower, and there is no tongue 
on either the anterior or the posterior border; the internal malleolus is less 
prominent, but more extended antero-posteriorly. The fibula has a very 
large head and a short, heavy shaft, which differs from that of Nesodon 
in being nearly straight and laterally compressed, rather than trihedral 
in form, and is without an interosseous crest. The wide space between 
the tibia and fibula is due to the great expansion of the ends of the former, 
especially of the proximal end of the tibia and the distal ends of both 
bones. The distal end of the fibula is relatively very much heavier than 
in Nesodon , but otherwise of similar shape, though the facets for the cal- 
caneum and astragalus, which in the Santa Cruz genus are connected 
only near the dorsal angle, are continuous throughout in Toxodon. 
The pes (PI. XXV, figs. 9-12) is remarkably small in proportion to the 
length of the limb-bones and the size of the animal generally ; it is even 
much smaller than the manus. The tarsus is not interlocking, thus retain- 
ing a more primitive character than the carpus. 
The astragalus is both short and narrow and has but a feebly grooved 
trochlea, which is asymmetrical, the external condyle being higher and 
narrower and having a sharper and more angular border than the internal. 
On the outer condyle the articular surface passes uninterruptedly into the 
facet for the fibula and that of the inner condyle passes in the same way 
into the facet for the internal malleolus of the tibia, this latter facet extend- 
ing into a shallow concavity on the neck, into which the malleolus is re- 
ceived, when the foot is strongly flexed upon the leg. The neck is 
extremely short and extends obliquely inward, allowing the outer portion 
of the trochlea to overhang the distal end of the calcaneum, though with- 
out any contact with the latter. The head is moderately convex in both 
directions and articulates with the navicular only, being widely separated 
from the cuboid. The external facet for the calcaneum is long, narrow 
and concave ; for most of its length it is separate from the fibular facet, 
but becomes confluent with it distally ; separated from the external cal- 
caneal facet by a broad and very deep sulcus is the surface for the sus- 
tentaculum, which is nearly plane and is broad proximally, narrowing 
distally to its junction with the navicular facet on the head. 
