20 
PATAGONIAN EXPEDITIONS : PALEONTOLOGY. 
cavity. The dilatation is not confined to the squamosal but extends also 
to the mastoid, with which the former is closely fused, resulting in the in- 
clusion of the external auditory meatus in a puffy mass of bone extending 
from the postglenoid process to the base of the paroccipital. 
A view of the upper surface of the skull (PI. Ill, fig. 2) shows that the 
nasals are long, broad behind, where they are in contact with the frontals, 
but tapering anteriorly to round points, which project slightly beyond the 
terminal narial opening. The nasals are convex in cross section anteri- 
orly, but are excavated longitudinally farther back, producing a sigmoid 
cross-section. As previously explained, the nasals are but slightly sup- 
ported on either side by the maxillary and premaxillary. 
A more or less elongated frontal process extends between the nasal and 
maxillary, while a second process, in contact with the lachrymal, is applied 
to the outer surface of the ascending process of the maxillary, excluding 
the latter from the orbit. The interorbital tract is approximately plane 
(in some specimens slightly concave, but this may be due to crushing). 
It is slightly elevated in the region of the ascending maxillary processes 
and sometimes also along the line of the interfrontal suture, which is more 
or less persistent. The temporal ridges are low, but sharply defined, and 
converge rapidly to form a low but strong sagittal crest. The postorbital 
processes are long and pointed, the posterior border coinciding with the 
temporal ridges. Just back of these processes the brain case is greatly 
constricted, but rapidly expands posteriorly, and in P. attenuatum superi- 
orly. The sagittal and lambdoidal crests join each other at a right angle. 
The latter bifurcates, sending one branch forward as the superior bound- 
ary of the temporal process of the squamosal and the other downward 
and forward over the inion to the base of the paroccipital process, bound- 
ding the lateral expanse of the distended squamosal and mastoid (PI. Ill, 
fig. 2). The temporal fossa is continued backward to the lambdoidal 
crest as a deep trough floored by the parietal and squamosal. Many 
foramina pierce the latter element, unlike Hegetotherimn, in which most of 
the foramina in this region perforate the parietal. 
On the back of the skull (PI. Ill, fig. 4) there is a large expanse of 
cancellous squamosal and mastoid, but so completely are these elements 
fused that the suture between them is not apparent. The supra- and 
exoccipitals are also fused. Dorsally, the supraoccipital is expanded, 
overlapping the mastoid and supporting prominent tubercles for the recti 
