Of the pelvis, the ilium is known as a broad, heavy bone with the acetabulum facing down. The hind limb is considerably longer than the front, and approximately pillar-like. The femur, as compared with the humerus, is quite a little longer, though, as femora go, it is not a long bone. The rounded sessile head stands high above the blunt, thick greater trochanter; the digital fossa is barely indicated; the rotular trochlea is short; the two condyles are subequal in size and set close together. The patella is short and nodular. The tibia is short and very 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 differing from all the others described in having pm. 1 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. 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. 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 in lower jaw of P. sorondoi. P. pluteum is based on three lower teeth of smaller size than the typical P. sorondoi, but the difference is small and there are no structural features accompanying it; so I consider it simply a smaller individual of P. sorondoi. In the generic discussion, Parapyrotherium planum was also assigned to P. sorondoi.

Pyrotherium romeri Ameghino

This species is characterized by the presence of pm. 1 which is absent in other species. The tooth is of fair size, two-rooted, narrow in front and has a narrow rim of enamel around it; and measures 22 mm. long by 14 mm. wide. The second premolar is 30 mm. long by 33 mm. wide. A lower tush is also associated with these two teeth and is of smaller size than in the following species, being at the alveolus border 40 mm. by 29 mm. in cross section.

Pyrotherium sorondoi Ameghino