From the foregoing chart and the comparative figures of the upper and lower dentitions, the variety and at the same time the homogeneity of the Typotheria is evident. The gnawing front teeth resemble those of rodents, especially in the genera where the enamel is lacking on all but the front face, but this is entirely a parallelism and there is no evident phylogenetic relationship. As to affinities with the Hyracoidea, Sinclair[12] has carefully balanced them and finds so little in common between the two groups that he makes them a separate suborder. I find certain features in common, like the lophiodont dentition with the tendency toward hypsodont incisors, the inflation of the tympanic and the extension of this up into the periotic region, and the general arrangement of the basicranial foramena. On the other hand, there are also numerous features in common with the Toxodonts, and several peculiar to the group, so that I would feel that all the Notungulates are descended from the Hyracoidea, and this group has developed its peculiarities in South America, retaining however a little more of the hyracoid aspect.

Fig. 24. Comparative series of upper dentitions
of Deseado and Santa Cruz Typotheria;

a,Archaeohyrax patagonicus;
b,Hegetotherium mirabile;
c,Prosotherium garzoni;
d,Pachrukhos moyani;
e,Archaeophylus patrius;
f,Interatherium extensum;
g,Protypotherium australe;
h,Argyrohyrax proavus;
i,Eutrachytherus spegazzinianus—
all natural size.

Fig. 25. Comparative series of lower dentitions
of Deseado and Santa Cruz Typotheria;

a,Archaeohyrax patagonicus;
b,egetotherium mirabile;
c,Prosotherium garzoni;
d,Pachyrukhos moyani;
e,Interatherium sp.;
f,Protypotherium australe;
g,Plagiarthrus clivus;
h,Eutrachytherus spegazzinianus;
i,Isoproedrium solitatium—
all natural size.

The Archaeohyracidae are the most primitive of the Deseado forms, but as all the families are already separated before this time the Deseado genera can not be considered as the ancestral ones, though they seem to have retained more of the primitive features.

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].