1 68 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 as in Palaeomastodon. The palatal bones are slender 

 in front, and broaden toward the rear, again, as in 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 



