168 THE DESEADO FORMATION OF PATAGONIA 
panic portion, completely inclosing the opening of the 
ear and crowding the tympanic from being exposed on 
the side of the skull. There is a very short paroccipital 
process, and this posterior portion of the squamosum is 
the part which resembles that of elephants, hyracoids 
and, to some extent, Toxodontia. There is, however, no 
cavity in the squamosum as in toxodonts generally. The 
tympanic bulla is small, but little swollen, and hollow. 
It is quite exactly like that of probocideans. The basi- 
occipital is fused to the exoccipitals. The occipital con- 
dyles are very high above the plane of the teeth, are set 
wide apart, and are cylindrical bosses which would not 
allow a free movement of the head laterally, but only in 
the up and down direction. This last is again a feature of 
the elephants. The pterygoid bone is greatly enlarged 
to compensate for the bend in the basicranial axis, and the 
pterygoids, together with the alisphenoids, make broad 
plates bounding either side of the posterior nasal chamber, 
exactly asin Palaeomastodon. The palatal bones are slender 
in front, and broaden toward the rear, again, asin elephants. 
On the interior of the brain case is the cavity for the 
brain which indicates that this organ was of diminutive 
size, measuring about 150 mm. in length by 50 mm. in 
width at the widest part. It indicates a brain with very 
small cerebral hemispheres, which, however, had a swollen 
posterior margin, a larger cerebellum, and a wide medulla 
oblongata. The impression which I obtained of this brain 
is strikingly like that given for Palaeomastodon. On either 
side of the brain cavity are a couple of vacuities, apparently 
for lightening the weight of the skull. At the inner end 
of the auditory meatus is a large ear chamber, divided 
into a smaller anterior or cochlear portion, and into a 
larger posterior ear chamber proper. 
In figure 109, I have placed a diagram of the base of 
the skull of Pyrotherium, along beside that of Palaeomas- 
todon, for comparison of the basicranial foramena. The 
