This group is composed of large, long limbed creatures, with a highly specialized dentition, in which the canines of the upper jaw are developed into great curved tushes, resembling those of _Pyrotherium_; while the canines of the lower jaw are compressed in the antero-posterior diameter and protrude laterally, like those of pigs. Upper premolars 1 and 2 are reduced or lacking, while pm. 3 and 4 are also reduced, but usually retained. The upper molars are brachydont, and have a crown very like that of the molars of homalodontotheres. The lower incisors are small, proclivous, and set at intervals around the broad semicircle of the front of the fused lower jaws. The lower canines are permanently growing teeth, smaller than the upper canines, project laterally, and have the tips recurved. Premolars 1 and 2 are usually lacking, pm. 3 more or less reduced, and pm. 4 is a normal, short, molariform grinder. The lower molars have the same basal pattern as in _Toxodonta_, the crown carrying two crescents with a plump pillar in the basin of the posterior crescent, the pillar, however, being situated far forward near the anterior horn of the rear crescent. Lydekker made an order _Astrapotheria_ including the _Astrapotheria_ and _Homalodontotheria_, but as the dentition of the two groups is so different, because of the enormous enlargement of the frontal region, and because of the reduction of the premolars, I am convinced that these two groups represent totally divergent lines of development; and I have therefore made each of the groups a separate suborder. Ameghino has described several genera, which make a progressive series and show a constantly progressive variation as far as they are known. GENUS FORMATION FORMULA Albertogaudryi Casamayor ? 1 ? 3 Post, inner and ------- post. isolated. ? 1 ? 3 median. cusps isolated. Astraponotus Astraponotus ? 1 2 3 Post. inner cusp, ------- united with wall ? 1 ? 3 making small lobe. Parastrapotherium Deseado and Colpodon ? 1 2 3 Post. lobe large, ------- also a strong 3 1 2 3 crista. Astrapothericulus Astrapothericulus ? 1 2 3 ------- 3 1 2 3 Astrapotherium Santa Cruz 3 1 2 3 ------- 3 1 1 3