160 THE DESEADO FORMATION OF PATAGONIA 
last molar expand into wide plates, just as in Palaeomas- 
todon (and in no other groups), having the postpalatine 
foramen opposite or behind the last molar. 
The posttympanic region of the squamosum is modified 
so that this process unites with the anterior squamosal 
region to crowd out more or less completely the tympanic 
bone where it should surround the auditory meatus. 
This feature is common to the elephants, the hyracoids, and 
the toxodonts, so that I consider it a primitive feature 
indicative of the ultimate common ancestry of these 
groups. The tympanic bulla can be compared with that 
of elephants closely, and has much in common with that 
of toxodonts, but in this last group the tympanic is much 
more highly developed. 
The premaxilla bone in Pyrotherium is crowded out, 
so that it makes no part of the palate, which is a character 
of elephants, and in contrast to toxodonts or other groups 
which have been mentioned. There are two antorbital 
openings as in elephants, and a feature not common, 
though not unknown. On either side of the brain case 
are cellular spaces with intercellular lamellae, which are so 
characteristic of elephants; a confirmatory feature, though 
in itself not conclusive. \ 
The foramena on the base of the cranium are similar 
to those on the base of the cranium of elephants, though 
there are some variations, as for instance, the exoccipital 
foramen, is isolated in Pyrotherium, but fused with the 
posterior lacerum foramen in elephants, and other slight 
variations in position; but, on the whole, the foramena 
of Pyrotherium are much closer to those of elephants than 
of any other group. There is also much in common 
with toxodonts and with hyracoids, as would be expected 
if they have a common ancestry. There is no suggestion 
of a marsupial arrangement as would be necessary if related 
to Diprotodon. 
