58 THE DESEADO FORMATION OF PATAGONIA 
that he makes them a separate suborder. I find certain 
features in common, like the lophodont 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 pecu- 
liar to the group, so that I would feel that all the Notun- 
gulates are descended from the /Tyracoidea, and this group 
has developed its peculiarities in South America, retaining 
however a little more of the hyracoid aspect. 
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 Interatherudae represent an offshoot line of develop- — 
ment 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 [ege- 
totheriidae 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. Propachy- 
rucos 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 develop- 
ment 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 Eutrachy- 
theridae 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, 
