Ameghino puts most of the forms in the family _Cephalomidae_, which he considers ancestral to _Hystricomorpha_ in general. I feel, however, that it is better to assign the Deseado genera to the families which have persisted until recent times, as Scott and Ameghino, in another place, have done. There are six living families, four of which Scott found already represented in the Santa Cruz. Two of these clearly may be continued back into the Deseado, the _Erethizontidae_, and the _Chinchillidae_, nothing as yet having been found to represent the Santa Cruz families _Cavidae_ and _Octodontidae_. Chinchillidae In the Deseado, this family is represented by the genera _Cephalomys_, _Scotamys_, and possibly _Litodontomys_. _Cephalomys_ is very abundant and seems to be ancestral to _Perimys_ of the Santa Cruz; _Scotamys_ is relatively rare but seems to be ancestral to _Scotaeumys_; while _Litodontomys_ is also rare and as far as I can see without a successor. =Cephalomys= Ameghino Cephalomys Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 494. This is the common genus of the Deseado, over three-fourths of the specimens of rodents found belonging to one of its three species. Its dental characters mark it clearly. All the premolars and molars are rooted, though the crown is incipiently hypsodont, as much so as in any rodent of this period. The incisors are moderately large with the anterior face slightly convex, and the antero-posterior diameter comparing with the transverse diameter as 3 does to 2. The interval between the incisor and premolar 4 is moderate, indicating a short snout. Each lower molar consists of two transverse laminae separated from each other by an internal and an external infolding, both of which approach the median line but do not meet, a narrow, longitudinal bar separating the folds and connecting the anterior and posterior laminae. On the inner side, the posterior lamina has a furrow extending to the middle of the tooth, but only sinking into the crown about a fourth of its height, so that, with wear, it appears first as a bay, later as a pit, and finally disappears. In general it will be found only on molar 3, and may be wanting there on old individuals. On an unworn tooth, there occurs, on the inner side of the anterior lamina, a rudimentary pit corresponding to the one on the posterior lamina, but of much less depth, so that it is only occasionally seen, and that only on a very slightly worn tooth. The premolar differs from the foregoing in having a small median column on the anterior face of the anterior lamina.