In the Deseado, this family is scantily represented as compared with the rich fauna, both as to species and numbers of individuals in the Santa Cruz, but of the four chief genera of the Santa Cruz, three have been found, though the remains are very fragmentary. They are the genera _Eoprototherium_, belonging to the _Prototherium_ series, _Deuterotherium_ belonging to the _Thaotherium_ series, and _Notodiaphorus_ representing the _Diadiaphorus_ series. The following table will give what is known in comparing the two series. PERIOD UPPER MOLARS NASALS PES Proterotherium Santa Cruz metaconule present normal tridactyl protoconule and protocone separate Eoproterotherium Deseado metaconule present protoconule and protocone separate Licaphrium Santa Cruz metaconule present normal tridactyl Diadiaphorus Santa Cruz metaconule present short tridactyl protoconule and protocone fused Notodiaphorus Deseado tridactyl Thaotherium Santa Cruz metaconule lacking normal monodactyl protoconule and protocone separate Deuterotherium Deseado metaconule lacking otoconule and protocone separate =Eoproterotherium= Ameghino Eoproterotherium Amegh., 1904, Anal. Mus. Nac. B. A., ser. 3, t. 3, p. 441. The genus is founded on single teeth of the upper molar series, which, except for size, are very like those of _Proterotherium_. Limbs, etc., are unknown, so that this genus is simply a carrying back of the _Proterotherium_ line into the Deseado. We found no teeth of this form, but one species has been described, _E. inaequifacies_, of which I reproduce Ameghino’s figure compared with _Proterotherium_, which shows this species to have the metaconule better developed. [Illustration: Fig. 5. _A_, Eoproterotherium inaequifacies, third upper molar—natural size; _C_, Protherotherium karaikense, third upper molar—natural size, after Ameghino.] =Notodiaphorus= gen. nov. The basis of this genus is particularly a hind limb found associated which is much less developed than the Santa Cruz genus _Diadiaphorus_ to which it is most nearly related. These two genera are unique in having the ectal facet on the astragulus developed in two planes so that it appears as a deep notch. In the case of the new genus the toes are almost equal in size, giving us a stage in the development of this three-toed form which is much more primitive than the well-known Santa Cruz genus. =Notodiaphorus crassus= sp. nov.