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. 



