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 P. romeri Amegh., 1889, Act. Acad. Nac. Cienc. Cordoba, t. VI, p. 618. P. romeri Lydekker, in part, 1894, Anal. Mus. La Plata, Palaeontologia Argen., t. III, supplement, p. 4. P. romeroi Amegh., 1894, Bol. Inst. Geog. Argen., t. 15, p. 612. P. romeroi Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 442. P. romeroi Amegh., 1902, Anal. Mus. Nac. Buenos Aires, ser. 3, t. 1, p. 32. P. romeri Gaudry, in part, 1909, Anal. Paleontologie, t. 4, p. 21. 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 P. sorondoi Amegh., 1894, Bol. Inst. Geog. Argen., t. 15, p. 613. P. sorondoi Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 443. P. sorondoi Amegh., 1902, Anal. Mus. Nac. Buenos Aires, ser. 3, t. 1, p. 30. P. sorondoi Amegh., 1906, Anal. Mus. Nac. Buenos Aires, ser. 3, t. 8, p. 331. P. romeri Lydekker, in part, 1894, Anal. Mus. La Plata, Paleontologia Argentina, t. III, supplement, p. 4. P. romeri Gaudry, in part, 1909, Anal. Paleontologie, t. 4, p. 21.