This family is distinguished, first, by the complete dental series in which none of the anterior teeth are developed into tushes; by the nasals being shortened, apparently in connection with the development of a proboscis; by its long neck; and by its feet being permanently tridactyl, all the three toes being equally developed. In the Deseado it is infrequent, but to it Ameghino has assigned two genera; _Protheosodon_, which he describes as similar to _Theosodon_, but which I find much nearer to the Casamayor representatives of this family, such as _Lambdaconus_, though it doubtless belongs to the series which is represented in the Santa Cruz by _Theosodon_. He has also made a second genus, _Conioptotherium_, which represents a large Macrauchenid, equal in size to _Theosodon_. This genus is based on the calcaleum and astragulus and seems to be rare. =Protheosodon= Ameghino Protheosodon, Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 453. Protheosodon, Amegh., 1904, Anal. Mus. Nac. B.A., ser. 3, t. 3, p. 421. This genus was founded on an upper second molar and the fourth premolar. I figure m. 2, and it will be seen that they represent a form little specialized, resembling in the low crowns, plump cusps, and presence of both protoconule and metaconule, the Casamayor types, such as _Lambdaconus_ or _Didolodus_, rather than the advanced type like the Santa Cruz genus, _Theosodon_. We found a specimen with the lower jaws complete and with the hind limb complete, which, I am confident, is the same form, though I can not duplicate any tooth, for we found no upper teeth; but in size they agree with _Protheosodon_, also in the primitive character; and, were one from the lower teeth to postulate the upper, they would be just such as Ameghino has described under the name _Protheosodon_. Therefore I have assigned my material to this genus and species. It adds to the genus characters the fact that this form had a shorter back, relatively as well as actually, than _Theosodon_; that the hind limb, at least, was much heavier and also shorter than that of _Theosodon_, especially in the metatarsal region where relatively the elements are only about half as long. The pes is of the same character as in _Theosodon_, but again relatively much shorter. I believe in _Protheosodon_ we have to do with a form intermediate between _Lambdaconus_ and _Theosodon_, and nearer to the former. =Protheosodon coniferus= Ameghino. P. coniferus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 453.