Fig. 47. Cast of the brain; A, from above; B, from the side—natural size. Cer, cerebellum; F, frontal lobe; H, hippocampal lobe; Oct, occipital lobe; Op, optic nerve; T, temporal lobe; x, cast of cavity in squamosal bone; Na, case of interior of nasal cavity.
Just behind the hippocampal lobes, and connecting with the cerebellum appear two striking projections. These represent the two large cavities in the upper part of the squamosum which are so characteristic of typotheres. The two large cavities clearly opened into the brain case by a broad connection which is especially wide at the lower ends. I find no traces of a connection with the bulla as described by Sinclair for Pachyrukhos. Further down are two small knobs apparently also representing cavities in the squamosum, and also connected with the brain case. In considering the brain these should be overlooked; but they doubtless represent some nervous function to which I have as yet no clue.
Ameghino considered that Eutrachytherus was the connecting link between Archaeohyrax and Typotherium. I feel that this genus is too highly specialized to be a connecting form, though it doubtless belongs to the series which ends in Typotherium; and such a form as Argyrohyrax is more likely to be the really ancestral form.
Two species are described, E. spegazzinianus, and E. conturbatus, which is about 15% smaller. Our collection offers a third species, E. grandis, which is nearly 50% larger than the first named species.
Eutrachytherus spegazzinianus Ameghino
Trachytherus spegazzinianus Amegh., 1889, Act. Acad. Nac. Cienc. Cordoba, t. VI, p. 919.
Trachytherus spegazzinianus Lydekker, 1894, Anal. Mus. La Plata, pt. 3, p. 2.
Trachytherus spegazzinianus Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 622.
Eutrachytherus spegazzinianus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 428.
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.
