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PATAGONIAN EXPEDITIONS : PALAEONTOLOGY . 
arrangement of the several elements. The delicate zygomatic process 
of the squamosal is not dilated, as in Hegetotherium , but the inflation of 
the mastoid is proportionately much greater than in any of the other 
genera, lodging a large spherical cavity communicating by a canal with 
the tympanic chamber (PL X, fig. 4). 
In superior view (PI. X, fig. 2) the long, blunt-pointed nasals are seen 
to increase greatly in width posteriorly, receiving a broad tongue of 
the frontals between them. They are separated from contact with the 
lachrymal by a long, narrow bar of the maxillary. The nasals are convex 
in cross-section anteriorly, but flatten out posteriorly. The interorbital 
tract is approximately plane transversely, the interfrontal suture persist- 
ing. Unlike Hegetotherium , the postorbital processes are slender and 
spine-like. No marked constriction of the brain-case succeeds them. 
The temporal ridges are exceedingly slight, appearing as faint lyrate 
crests, originating just back of the postorbital processes and converging 
posteriorly, but not uniting. 
The most prominent feature of the back of the skull is the greatly dis- 
tended mastoid bulla, which projects posteriorly beyond the condyles. 
These bullae show but slight contact with the squamosal anteriorly and 
are separated from the parietal by a groove opening into the cerebral 
chamber. Internally, they are hollow and, as in Hegetotherium , are con- 
nected by a canal with the tympanic cavity (PI. X, fig. 4). The sutures 
between the various occipital elements can no longer be distinguished. 
Dorsally, the supraoccipital appears on the upper surface of the skull 
between the mastoid bullae. Posteriorly, there is a strong median con- 
vexity for lodgment of the vermis of the cerebellum, bounded on either 
side by a broad longitudinal concavity. The degree of transverse con- 
striction of the occipital in the region of the mastoid foramen is propor- 
tionately much less than in Hegetotherium. The foramen magnum and 
occipital condyles are similar to those of the last named genus. The 
paroccipital processes are quite long, increasing greatly in antero-posterior 
diameter a short distance below their bases, where they form a thin plate, 
from the posterior margin of which a narrow, transversely compressed 
process is continued downward, gradually curving forward toward the 
tip (PI. X, figs. 3, 4). 
The palatal surface of the skull recalls that of Hegetotherium. The 
tooth-rows are slightly arched, inclosing an area deeply concave both 
