170 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 Pyrothertum 
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 stylomastoid 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. 
