50 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
of the alveolar border between the second incisor and the second pre- 
molar (PI. VIII, fig. 1 6). The facial expanse of the maxillary is broadly 
concave both antero-posteriorly and transversely. It is continued upward 
as a robust, V-shaped bar inclosed in a deep notch in the frontal, widely 
separating the nasal from the lachrymal. The maxillary is extensively 
involved in the anterior and inferior margin of the orbit and in the tem- 
poral arch, where it extends as far back as the posterior margin of the 
glenoid cavity, forming with the malar a strong buttress preventing lat- 
eral dislocation of the mandible. A large descending maxillary process 
is developed beneath the orbit and the whole lower surface of the temporal 
process of this bone is deeply pitted for muscular attachment. The orbits 
are circular, quite prominent and widely open posteriorly. They are 
bounded anteriorly and interiorly by the maxillary, the orbital portion of 
which rises above the facial tract as a vertical plate (PI. VIII, figs. 16, 17). 
The lachrymal is entirely orbital, a “ lachrymal ” tubercle being developed 
from the maxillary. Almost directly beneath this tubercle, the circular 
infraorbital foramen perforates the maxillary at the junction of its orbital 
and facial portions. Posteriorly, the orbit is bounded by the anterior 
extremities of the malar and squamosal. These elements have the same 
mutual relations as in Protypotherium , but the malar is proportionately 
heavier than in the latter genus. The squamosal is distended and filled 
with cancellae, the distention involving also the mastoid. 
In superior view (PI. VIII, fig. 17; text figs. 11, 12) the nasals are seen 
to be very broad, with square-cut tips and either straight (/. extension) 
or curved fronto-nasal suture (/. robustmn). They are not firmly united 
with the premaxillae and maxillae and are frequently crushed down into the 
narial chamber. A broad, V-shaped maxillary process widely separates 
the nasal from the lachrymal. The interorbital tract is flat, with persistent 
median suture. The postorbital processes are short and robust, with blunt 
points. Their posterior borders give rise to the temporal ridges, which 
are heavy and converge rapidly at an acute angle to form a prominent 
sagittal crest in I. robustum and /. extension , while in I. excavation they 
are lyrate, lower and converge much farther back on the parietal to form 
a short low crest. Back of the postorbital processes, the brain case attains 
its maximum constriction, but expands rapidly posteriorly. The lamb- 
doidal and sagittal crests join at a right angle, inclosing, with the superior 
border of the temporal process of the squamosal, a deep temporal fossa, 
