The relatively small olfactory lobes are entirely beneath the frontal lobes and are seen only on the side view, but as there is a large hippocampal lobe behind them, it would only seem proper to attribute to these animals a well-developed sense of smell. The frontal lobes are unexpectedly large, and are not clearly bounded off from the parietal lobes. The occipital lobes are also well developed and make a large portion of the backward extension of the cerebrum. The large size of this area, together with the fact that the optic nerves are large, indicates a good visual development. The temporal lobes are also large and extend well down on either side. And, finally, below all the others, come the swollen hippocampal lobes which complete this large cerebrum. The cerebellum is small having neither considerable width or height, and being overlapped by the cerebrum. The optic nerves are represented by large projections leaving the twixt-brain well forward under the forebrain. The medulla is not clearly marked except to show that it, too, was of fair size. [Illustration: 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.