1 70 THE DESEADO FORMATION OF PATAGONIA 



skull of Palaeomastodon is somewhat more elongated, 

 especially in the posterior part. In both, there are two 

 antorbital foramena; the postpalatal foramena of Pyro- 

 therium are a trifle further back, but this palatal region in 

 both is of the same type which is peculiar to elephants 

 and Pyrotherium. In Pyrotherium the condylar foramen 

 is separate, while in elephants it is fused in with the fora- 

 men lacerum posterior. This latter foramen in both cases 

 is situated just back of the tympanic, and in Pyrotherium 

 is of considerably larger size than in Palaeomastodon. 

 The foramen lacerum medium is in front of the tympanic 

 and in Pyrotherium appears considerably larger, mostly 

 because it is under the margin of the tympanic in Palaeo- 

 mastodon. The foramen for the internal common carotid 

 in Palaeomastodon pierces the tympanic bone just to the 

 inside of the middle line, while in Pyrotherium it is on the 

 outer margin of the tympanic. The Eustachian canal is 

 on the external border of the tympanic in both cases, but in 

 Pyrotherium it is further back. The foramen ovale of 

 Palaeomastodon is in the posterior part of the alisphenoid 

 bone, but with the shorter alisphenoid of Pyrotherium^ 

 this foramen is pushed back to the posterior margin of 

 the bone. In both cases, the alisphenoidal canal starts 

 under the base of the fused alisphenoid and pterygiod, 

 and opens into the orbit. The stylornastoid foramen of 

 Pyrotherium is situated further out than in the case of 

 Palaeomastodon. The fusion of the postympanic portion 

 of the squamosum is, in Palaeomastodon, much further 

 advanced than in Pyrotherium, so that the passage to the 

 ear is not apparent in the basal view of the former, but 

 makes a considerable notch on the under side of the skull 

 of Pyrotherium. 



The mandibles are excessively thick and heavy, being 

 united at the symphysis, which extends back to the front of 

 the second molar. The ascending rami are prolonged back- 

 ward, but do not rise above the level of the articulation. 



