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 

 forameha. 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 Ilyracoidea, 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 Inter cither iidae 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 

 Inter cither turn and Proty pother ium. In the family Hege- 

 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, 



