Suborder Pyrotheria This suborder was established by Ameghino to receive the peculiar genus _Pyrotherium_ and related forms. These animals are of large size, massive build, with narrow elongated skulls, in which the nasal opening is situated far back, as in animals with a proboscis; with a tiny brain case surrounded by cellular spaces; with the maxillae, palatines, pterygoids and alisphenoids developed downward, so that the palatal plane makes a strong angle with the basicranial plane; and with the occipital condyles high up on the back of the skull. Then the first and second upper incisors and the second lower incisors are developed into enormous tushes with enamel on the anterior sides only. The remaining incisors, the canines, upper premolar 1, and lower premolars 1 and 2 are wanting; the remaining premolars and the molars being developed into great quadrilateral grinders, each with two transverse crests. The neck is short, the limbs massive and short, especially the lower members of each limb, and the feet were probably five-toed. The relationship of these forms has been the subject of extended discussion: Ameghino seeing in this genus the ancestors of the _Proboscidea_, and comparing them with _Palaeomastodon_, _Dinotherium_ and _Barytherium_, even finding resemblances to _Diprotodon_; Gaudry concludes that they are not proboscidians; and others have suggested that they were specialized toxodonts. I have prepared the following table of comparisons with _Palaeomastodon_, a toxodont, and _Diprotodon_.