8o 
PATAGONIAN EXPEDITIONS ! PALAEONTOLOGY. 
face supports a facet, confluent with the crescent-shaped facet for articula- 
tion with the inner side of the fibula, which is in contact with the small, 
trapezoidal fibular facet, already described, when the foot is in extreme 
flexion. The inner process rises from the plantar margin of the astraga- 
lar body. Internally, the inner margin of the trochlea articulates with the 
tibial malleolus. The neck is long and directed rather more obliquely than 
in Protypotherium . The head is spherical, fitting into a cup-shaped de- 
pression in the navicular. On the plantar surface (PI. II, fig. 20) the sus- 
tentacular facet is reniform in outline, but is wider distally than proximally. 
Transversely, it is almost a plane surface, but in proximo-distal section is 
broadly S-shaped. The astragalar facet is dumbbell-shaped, constricted at 
the middle and wider at the ends. It is concave in proximo-distal section 
and plane transversely. 
The calcaneum (PI. II, figs. 15, 19) may be readily distinguished from 
that of Protypotherium by the small size of the fibular facet, which is hardly 
differentiated from the ectal facet, and by its extensive articulation with 
the navicular. The ectal facet is much longer and wider than in Protypo- 
therium. It is convex in proximo-distal section, approximately plane 
transversely and presents forward and inward. The sustentaculum is 
wedge-shaped, decreasing in width toward the free border, while in Protypo- 
therium it is slightly wider at the free border than at the base. The sus- 
tentacular facet is circular and concave in all dimensions, but especially 
proximo-distally. The surface for the cuboid (PI. II, fig. 15) is concave in 
dorso-plantar section, more so than in Protypotherium. Internally, it is 
confluent with a small, almost plane facet for the navicular, which is entirely 
wanting in Protypotherium. The tuber has much the same shape in both 
genera, but the rugosities at the free end are proportionately smaller in 
Hegetotherium . 
The navicular is deeply cupped proximally for articulation with the 
round head of the astragalus, while distally (PI. II, fig. 16) it supports 
two heart-shaped facets for the ecto- and mesocuneiforms. The facet for 
the ectocuneiform is plane in both dimensions ; that for the mesocunei- 
form is concave transversely and convex in the direction at right angles 
to this. On the inner side a small, semicircular, almost plane facet sup- 
ports the extremity of the internal cuneiform. An oval, plane facet on 
the outer side articulates with the cuboid. Another, for contact with the 
calcaneum, lies between the cuboidal facet and that for the head of the 
