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. romert, 
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 romerot) 
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. sorondot, which is somewhat larger 
than P. romeri, and lacks pm. 1 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. | can see in this only a large 
individual of P. sorondot. 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 
