264 PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 
The parietals are long and narrow, forming but little of the side-walls 
of the cranium. So far as I can make out, these bones are thin through- 
out their length and do not have the posterior thickness and massiveness 
which they have in Nesodon. In the fully adult animal the sagittal crest, 
which is very prominent, continues for the entire length of the parietals, 
but in the young skull it passes anteriorly into a narrow, triangular area, 
which broadens to the frontal suture. The anterior ends of the bones 
diverge less than in Nesodon and the frontals do not project nearly so far 
backward between these divergent ends. Along the squamosal suture 
on each side is a row of four or five large and conspicuous vascular 
foramina. 
As in Nesodon , the squamosals are very large and form the greater 
part of the cranial side-walls, but there are many differences of detail, 
especially in the shape and size of the various processes. The glenoid 
cavity is a saddle-shaped surface, the direction of which is rather oblique ; 
it is narrow and very slightly convex antero-posteriorly, broad and some- 
what concave transversely. Externally, this articular surface forms a low 
and barely perceptible protuberance, which is very much lower and less 
conspicuous than in the Santa Cruz Toxodonta. The space between the 
glenoid cavity and the postglenoid process is much narrower than in the 
latter and the postglenoid process is of an entirely different shape ; it is 
a long, freely projecting and rather massive process, broad and thick 
proximally, where it is extensively applied to the post-tympanic process, 
completely excluding the mastoid from the surface of the skull, but taper- 
ing and becoming styliform distally. In Nesodon and Adinotherium this 
process is a broad, heavy and swollen-looking plate, no part of which is 
free, but is closely united with the mastoid, which separates it from the 
post-tympanic. The glenoid foramen is a larger and more conspicuous 
opening than in the genera last named, and channels the postglenoid 
process. 
The post-tympanic process is of the same curious and anomalous char- 
acter as in all of the other Toxodontia, though each of the three suborders 
of this group has its own particular modification of the general plan. In 
Homalodontotherium the lateral exposure of the post-tympanic is much 
greater than in Adinotherium or Nesodon , forming the posterior portion 
of the zygomatic arch and its dorsal border being raised into a ridge, 
which is continuous with that of the zygomatic process on one side and 
