Most striking of all the typothere peculiarities, is the development of the first upper and lower incisor into permanently growing teeth, having the enamel reduced to the anterior side only, making thus a self-sharpening tooth similar to that of rodents. Such teeth are characteristic of gnawing forms and would indicate that the form lived, in at least a considerable part, on bark and twigs. In the eating of such food and breaking up the wood cells for the contained protoplasm and starch, an immense amount of chewing is involved, followed by a rapid wear of the molars. This is met, as is characteristic in rodents and grass eaters, by the development of first high-crowned, then permanently growing molars. In acquiring the permanently growing tooth, some of the irregularities of the crown are lost, others which are deep-seated enough to affect the tooth even to the root are maintained, so that especially the external and internal infoldings become a persistent part of the tooth, having been impressed into the dental papilla. A further supplement to the resistant character of the teeth is seen in the development, in the most advanced types, of a cement layer on the outside of the molars, a feature apparently also a part of permanently growing roots. The feet are generally those of a running type, but a single phylum has acquired the hopping habit. The above features seem to indicate a more special adaptation than grass feeding. From the aspect of the whole Deseado fauna, we would seem to be dealing with the inhabitants of an arid area, where bushes have, in part at least, replaced the grass. The typotheres seem to me to represent a part of the fauna which lived by gnawing the bark and eating the twigs and leaves of bushes. This does not preclude the eating of grass also, but I do not see how they would have developed all their peculiarities by eating grass alone. The rodents are of such insignificant size that they could hardly have monopolized this food supply, and the typotheres seem to have adjusted themselves to, and occupied the place of rabbits on our western plains; but went even farther in developing in great numbers and varieties. SYSTEMATIC DESCRIPTIONS =Archaeohyracidae= Ameghino This family is differentiated by the presence of enamel on all sides of the first incisor, by the unreduced condition of the lateral incisors, and by the small bulla of the mastoid. These are primitive features. Ameghino considered this family to belong to the hyracoids; but, as explained earlier, I believe them to be true _Typotheria_, though less specialized than the other families. =Archaeohyrax= Ameghino Archaeohyrax Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 431. This interesting genus is known by a complete skull found by Ameghino and of which we found no duplicates. I insert a reproduction of the side view of the skull, and the dentition is shown in fig. 24 a, and fig. 25 a. The dental formula is 3 1 4 3 -------. 3 1 4 3