HISTORY OF THE JTOXODONTIA 481 



transition of increasing size and complexity from the incisors 

 to the molars. A remarkal^le feature of this genus was the 

 deeply cleft form of the lower incisors, giving them a fork- 

 like shape, somewhat as in the modern Hyracoidea. The 

 ulna and radius in the fore-arm and the tibia and fibula in the 

 lower leg were separate, but the digits were already reduced to 

 four in each foot. This was one of the few Santa Cruz un- 

 gulates which possessed a long and heavy tail. The limbs 

 were relatively long and the feet were armed with such slender 

 hoofs that they looked almost like claws. The restoration 

 shows the animal to have had, like nearly all of the fTypo- 

 theria, a very rodent-like appearance, a likeness which may, 

 perhaps, be unduly increased by the form given to the ears. 



In the allied genus, ]Interathenu7n, from which the family 

 is named, the head was short, broad and deep, almost bullet- 

 like ; the first incisor was enlarged and chisel-shaped, and the 

 other incisors and the canines were much reduced in size. It 

 is an interesting fact, observed as yet only in this genus, but 

 probably true also of all the smaller members of the suborder 

 which had hypsodont teeth, that the milk-premolars were 

 rooted and comparatively low-crowned, while their permanent 

 successors were completely hypsodont and rootless. The 

 limbs were considerably shorter than those of \Protypotheriu7n 

 and the tail long and thick, except for which, the general 

 appearance of the skeleton suggests that of the modern ''co- 

 nies" or "klipdases" (Hyracoidea) of Africa and Syria, a sug- 

 gestion which Mr. Knight has followed in the drawing (Fig. 

 297, p. 636). 



This family was represented in the Deseado stage by a 

 genus (]Plagiarthrus) in which the teeth developed roots in 

 old age, but is not known from more ancient formations. 

 Their probable ancestors of the Eocene were ver}^ small ani- 

 mals, with brachyodont teeth, the premolars smaller and of 

 simpler pattern than the molars. The upper molars had a 

 continuous external wall, with indication of separate cusps, 

 2i 



