_Parapyrotherium_ is based on a small molar and a tush which Ameghino first described as _Pyrotherium planum_, later elevating the species to a genus, designated as _Parapyrotherium_, differentiated by the transverse crests being low and the valley at either end being blocked by an intertubercular ridge. Gaudry considered that this genus represented either the milk teeth of _P. romeri_, or a variation of that species. I can not see the basis of a new genus in the material. The genus _Richardoweni_ is based on half of a molar, which has the transverse crest interrupted in the middle. Too little is known of this form to base a valid genus or even to associate it with _Pyrotherium_. _Archaeolophus_ is founded on a small tush and part of an upper molar, also inadequate material for a genus. It is probably _Pyrotherium_. _Propyrotherium_ is a smaller form from the Astraponotus beds, apparently a good genus; the type species being _P. saxeum_, of considerable smaller size than any of the Deseado species. The distinctive features of the genus can not be given until more material is known. _Carlozitteli_ is based on a small form from the Casamayor with narrow molars. An incisiform tush is associated with the molar, which, if correctly associated, would indicate a wide deviation from _Pyrotherium_, and would probably be an ancestral form. A second species is reported from the lower Deseado beds, but I am a little skeptical as to the horizon. =Pyrotherium= Ameghino Pyrotherium Amegh., 1889, Actas Acad. Nac. Cienc. Cordoba, t. VI, p. 617. Pyrotherium Lydekker, 1894, Anal. Mus. La Plata, Palaeontologia Argentina pt. 3, p. 4. Pyrotherium Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 609. Pyrotherium Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 441. Pyrotherium Amegh., 1902, Anal. Mus. Nac. Buenos Aires, ser. 3, t. 1, p. 19-43. Pyrotherium Amegh., 1902, Anal. Mus. Nac. Buenos Aires, ser. 3, t. 1, p. 223-4. Pyrotherium Gaudry, 1909, Anal. Palaeontologie, t. 4, p. 1-28. Parapyrotherium Amegh., 1902, Anal. Mus. Nac. Buenos Aires, ser. 3, t. 1, p. 29. The type species of the genus is _P. romeri_, which is however a rare species, most of the material and the best known belonging to _P. sorondoi_. The Amherst Collection contains a skull, complete except that the top of the brain case is crushed in and the parietals lost; a second skull with the full upper dentition but lacking the cranium; four lower jaws; two isolated tushes; the atlas, axis, and cervicals 3 and 4; the humerus; the proximal end of the femur; and part of the front foot; all from the Chico del Chubut west of Puerto Visser. Gaudry had upper and lower dentition and the fore and hind limbs except the feet. Ameghino described the upper and lower dentitions and a fore foot, so that with our material we now have a basis for a fairly complete discussion, the vertebral column being the major part which is still lacking.