The _Interatheriidae_ represent an offshoot line of development in which the incisors are not much enlarged and the infoldings of the teeth remain. The genus _Archaeophylus_ seems to be directly ancestral to the Santa Cruz genera _Interatherium_ and _Protypotherium_. In the family _Hegetotheriidae_ there is a strong tendency for the incisors to develop into very large gnawing teeth, while the lateral incisors, the canine and the first premolar, tend to drop out, and the molars become more simplified. _Propachyrucos_ seems to represent a hold over of the most primitive type of these. The _Prohegetotherium_ and _Hegetotherium_ have retained the less specialized feet and less advanced type of teeth, while _Prosotherium_ has tended to the development of the hopping mode of locomotion, which is attained in _Pachyrukhos_ later. There thus seem to be two series inside of this family. When the material is better known, it may be best to separate the two series. The _Eutrachytheridae_ have retained the complexity of molars united with a permanently growing incisor. They seem also to have developed into a series of comparatively large forms, which, as they have advanced, have developed a bifurcated fold on the inner side of the upper molars, which in its complete development makes the upper molars three-lobed, as is seen in the typical _Typotherium_, representing the end of the series up in the Pampean formation. These relationships may be expressed graphically as in fig. 26. [Illustration: Casamayor | | Deseado | | Santa Cruz | ----------+-+------------------+-+----------------+ | | Archaeohyrax | | | | | | | | | | Archaeophylus | | Protypotherium | | | | | Interatherium | | | | | | | | Propachyrucos | | | | | Prohegetotherium | | Hegetotherium | | | Prosotherium | | Pachyrukhos | | | Phanophilus | | | | | | | | | | Argyrohyrax | | | | | Eutrachytherus | | | | | Isopoedrium | | | Fig. 26. Phylogeny of the Deseado and Santa Cruz Genera of Typotheria.] ADAPTATIONS