174 THE DESEADO FORMATION OF PATAGONIA 



heavy. The fibula is free its entire length and is a rather 

 heavy bone. The astragulus is a lens-like bone with the 

 trochlea but slightly convex, and the navicular facet 

 directly below it, indicating a rectigrade foot. 



Ameghino established the following species, P. romeri, 

 P. sorondoi, P. giganteum, P. crassidens, P. trilophodon, 

 P. pluteum. This is a very considerable number of species 

 of such a large type to occupy a limited region. Gaudry 

 has lumped them all under species P. romeri. This, I 

 think, is too drastic and I would find at least two species. 

 It is true that there is great individual variation in such 

 large animals, due to age, food supplies and individual 

 vicissitudes; but where there is a difference of dental 

 formula or a structural divergence I should consider these 

 as specific in character. 



The type species is P. romeri (later spelled romeroi) 

 which was based on a first and second upper premolar and 

 an upper tush, all of smaller size than P. sorondoi and differ- 

 ing from all the others described in having pm. I present. 

 Gaudry suggests that this may be the milk dentition but 

 there is no evidence as yet to settle this, so I have left 

 this species standing. Most of the material found by 

 Ameghino, by Gaudry and by our party belongs to the 

 type described as P. sorondoi, which is somewhat larger 

 than P. romeri, and lacks pm. I in the upper jaw. This 

 then is the usual species and to it belongs most of what is 

 known. It varies some in size but the characters are 

 very uniform. P. giganteum is based on the root of a large 

 tush, 90 by 70 mm. in cross section, which Lydekker 

 associated with P. romeri, and which Ameghino later took 

 as the type of a new species. I can see in this only a large 

 individual of P. sorondoi. P. crassidens is based on a 

 last lower molar 90 by 80 mm. in diameter. It seems to 

 me to be an upper molar and no larger than m. 2 in either 

 of my skulls. P. trilophodon is based on a lower pm. 3, 

 which, in every way, resembles the corresponding tooth 



