In many ways, the family suggests _Nesodontidae_, and undoubtedly belongs to that series, if not directly ancestral. The lower molars are distinctly of the same type as in all the other toxodonts, but show a tendency to become hyposodont. The following genera have been assigned by Ameghino to the family. Some of them are based on very scant material and I have ventured to suggest in each case what disposition I have felt to be the proper one. =Leontinia=, the type genus, is described in detail on pages 109-115. =Scaphops= is based on a mandibular symphysis, which is wider than usual for _Leontinia_, and on a second upper incisor which is compressed. The species in the genus _Leontinia_ show a marked degree of variability, and I can see in this only individual variability, so that I place _Scaphops_ under _Leontinia_ and _S. grypus_, as a synonym of _L. gaudryi_. =Steniogenium= is based on a mandibular symphysis with roots only of the teeth. The incisors are proclivous and inc. 3 small. I consider this also as _Leontinia_, and the species _S. sclerops_ as a synonym of _L. oxyrhynca_, which I think is the female of _L. gaudryi_. =Ancylocoelus= is a valid genus, differentiated by its dental formula 3 0 4 3 -------, 3 0 3 3 the loss of the upper canine and the lower canine and first premolar distinguishing it from either _Leontinia_ or _Colpodon_. =Rodiotherium= is based on a mandibular symphysis which would indicate an animal with the same formula as the foregoing genus, differing only in that lower incisor 3 is large. This, to my mind, does not make a generic character, and at most the species, _R. armatum_, can only be considered an independent species belonging to the genus _Ancylocoelus_. =Loxocoelus= is a very questionable genus, based simply on an upper molar, which “is similar to that of _Homalodontotherium_, but more squared.” I feel that in regard to this genus it should stand as unknown until more material is found. In our collection, over twenty skulls and jaws belonging to this family turned up, but all clearly belong to two types, the typical _Leontinia gaudryi_, and some others in which the caniniform teeth are not so well developed, which are either _L. oxyrhynca_ or, as I believe, the females of _L. gaudryi_. It is this uniformity of the material which leads me to doubt the validity of the considerable number of genera which Ameghino has established, for I found on sectioning the teeth that between the little worn crown and the much worn one there was a marked difference in the appearance of the infoldings and in the development of the pits. =Leontinia= Ameghino Leontinia Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 647. Leontinia Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 469. Scaphops Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 475. Steniogenium Amegh., 1895, Inst. Geog. Argen., t. 15, p. 654. Steniogenium Amegh., 1897, Inst. Geog. Argen., t. 18, p. 475.