SINCLAIR I TYPOTHERIA OF THE SANTA CRUZ BEDS. 
79 
femur are approximately equal. Both are circular in outline. The inner 
is strongly concave from side to side and very slightly convex antero- 
posteriorly ; the outer is strongly convex from before backwards and plane 
transversely. The spine is inconspicuous. The shaft is irregularly tri- 
angular proximally, but in its central portion becomes laterally flattened, 
in contrast with Protypotherimn , in which this portion of the shaft is cir- 
cular in cross-section. A strong cnemial crest terminating in a prominent 
tubercle extends about one-third the length of the shaft. The shaft is 
strongly arched inward, the straight, slender fibula forming the chord of 
the arc. The shaft of this element is semi-circular in section, with the 
inner surface flat. Distally, both bones are firmly welded, the laterally 
expanded distal end of the tibia forming with the fibula a broad flat area 
on the anterior side of the symphysis and a deeply excavated depression 
on the posterior side. The internal malleolus is very long and deeply 
grooved postero-internally for the lodgement of tendons. The fibula is 
also produced distally, but does not extend so far as the internal malleolus. 
The surface for articulation with the astragalar trochlea (PI. II, fig. 2) is 
deeply sunk between them. The outer lip of the trochlea is lodged in a 
deep, V-shaped notch along the line of suture between tibia and fibula. 
The groove for the inner trochlear lip is U-shaped in transverse section 
and extends down the outer surface of the internal malleolus. The fibula 
supports three facets distally, a large, flat facet for articulation with the 
lateral surface of the body of the astragalus, an almost flat, trapezoidal 
facet in contact with the lateral process of the astragalar body, when the 
foot is in the position of extreme flexion, and a small, lemniscus-shaped 
facet for contact with the calcaneum. The groove for the peroneal tendons 
is well defined and the external malleolus prominent. 
No complete specimen of the pes is preserved, but by putting together 
parts of two specimens (Nos. 15,392 and 15,298) it has been possible to 
reconstruct an almost complete tarsus and the entire metatarsus (PI. II, 
fig. 19). The astragalar trochlea is wider than in Protypotherimn , but the 
margins are unequally developed, the external being high and sharp and 
the internal low and round. The former fits into the deep groove between 
the distal ends of the tibia and fibula. The trochlear surface is short and 
not produced as far backward as in Protypotherimn. The astragalar body 
supports two processes, as in Pachyrukhos. The external process rises 
from the distal outer corner of the body of the astragalus. Its dorsal sur- 
