This, the type species, is by far the commonest of the rodents, in fact of all the species in the Deseado, and we found forty-seven specimens on the Chico del Chubut River, west of Puerto Visser. In the species there is considerable variation in size for a rodent, but as there are intermediate specimens all the way between the extremes, and as the variation is mostly in the size of the fourth premolar, it does not seem proper to separate the material into more than one species. [Illustration: Fig. 118. Right upper premolar, molar series × 4/1.] [Illustration: Fig. 119. Left lower premolar, molar series; _A_, deciduous premolar 4; _B_, a little worn molar 1; _C_, series about half worn down; _a.l_, anterior lamina; _p.l._ posterior lamina; _i.f._, internal fold; _e.f._, external fold; _p.g._, furrow in posterior lamina; × 4/1.] In general, the form has relatively plump teeth, relatively heavier and thicker than in the other species. Usually the fourth premolar is but little larger than the molars, but, in this character, there is considerable variation. The following measurements give the range of size on the upper jaws: SPECIMEN SPECIMEN AMEGHINO’S 3109 3099 TYPE A SMALL A LARGE INDIVIDUAL INDIVIDUAL Upper premolar 4 to m. 3 12.5 mm. 13.5 mm. 13.5 mm. Upper premolar 4, length 3.5 mm. 4.5 mm. Each molar, length 3 mm. 3 mm. Each molar, width 2.75 mm. 3 mm. The lower jaw is low, heavy and rather short, the posterior part of the ramus being very thin, while the portion carrying the teeth is thick and heavy. A strong ridge extends along the inner side from just behind molar 3 to the base of the symphysis. As in the upper dentition, there are smaller and larger forms. SPECIMEN SPECIMEN AMEGHINO’S 3089 3058 TYPE A SMALL A LARGE INDIVIDUAL INDIVIDUAL Lower premolar 4 to m. 3 14 mm. 15 mm. 14.5 mm. Lower incisor 1 to pm. 4 7 mm. 9 mm. 7.8 mm. Height of mandible under pm. 4 7 mm. 8 mm. 7 mm. Length of deciduous pm. 4 5.5 mm. =Cephalomys plexus= Ameghino C. plexus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 494. In general, this species is similar to the foregoing, but is smaller in size and slenderer in proportions. Both the upper and lower fourth premolar tend to be considerably larger than the molars. The species was not nearly as abundant as _C. arcidens_, occurring but sixteen times in our collection. [Illustration: Fig. 120. Right palate showing premolar, molar series; external furrows appear as pits on molars 2 and 3, × 4/1.]