This species was founded on the anterior part of a skull with the full upper dentition. My specimen differs from Ameghino’s in having a tiny alveolus for the upper canine, the difference being due to my specimen being younger. The upper dentition is very characteristic. Incisor 1 is a powerful, deep-set, curved gnawing tooth, with a heavy layer of enamel on the anterior face, and none on the other sides; and is moderately beveled in the rear as a result of wear. The second and third incisors are much smaller, each having enamel on the outer face only, and with a marked tendency to become vestigal. The suture of the premaxilla comes to the base of inc. 3. There follows a short diastema, then a tiny alveolus for the canine, closely followed by the first premolar which is also small. The second premolar shows no inflexion. Beginning with the third premolar there is a strong inner inflexion, which in the fourth premolar and molars is bifurcated. The molars are considerably larger than the premolars, the second being the largest of the series. With each successive molar, the inflexion is wider, so that in m. 3 the tooth is divided into three lobes of nearly equal size. All premolars and molars are rootless, curved, and set so deep in the jaw that they almost meet in the median line. A typical molar measures 50 mm. in length, of which only 7-8 mm. project above the border of the jaw. All the back teeth are covered with a thick coating of cement. [Illustration: Fig. 48. E. spegazzinianus, right upper dentition—natural size.] The arrangement of the teeth of the lower jaw is shown in fig. 25 h., after Ameghino. The first and second incisors are greatly enlarged. Incisor 3 is lacking, and the canine vestigal. Pm. 1 is small and single-lobed, the succeeding premolars and molars being large and divided into two lobes by a deep external, and a shallow internal infolding. MEASUREMENTS Skull, width between the orbits 60 mm. Skull, width across the postorbital processes 88 mm. Skull, height from m. 2 to top of frontal 77 mm. Skull, width of palate opposite inc. 2 42 mm. Skull, width of palate opposite m. 2 62 mm. Dentition, length inc. 1 to m. 3 140 mm. Dentition, incisor 1 ant.-post. length 11 mm. width 16 mm. Dentition, pm. 3 length 11 mm. width 13 mm. Dentition, m. 2 length 22 mm. width 16 mm. =Eutrachytherus conturbatus= Ameghino Trachytherus conturbatus Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 623. Eutrachytherus conturbatus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 429.