378 



LAND MAMMALS IN THE WESTERN HEMISPHERE 



has left no record of its Eocene or Oligocene terrestrial life. 

 On the other hand, no one can imagine that everything that 

 can be known of the mammals of the middle and lower White 

 River has already been learned, and at any time the sought- 

 for ancestor of ]Leptauchenia may be found in those beds. 



The fourth phylum may be regarded as the main or central 

 stem of the family and was the one which underwent the least 

 change, though it probably gave rise to all the other phyla. 



Fig. 201.- 



Leptauchenia nitida, upper White River. Restored from a skeleton in the 

 Museum of Princeton University. 



which branched off from it at various stages in its history. 

 This series terminated in the middle Miocene and comprised 

 several genera, all very much alike, in the lower stages of that 

 epoch. One of these genera ('\Mesoreodon) displayed a very 

 remarkable peculiarity of structure in the ossification of the 

 great cartilage of the larynx, which seems to point to the pos- 

 session of uncommon vocal powers. It is impossible to say 

 whether this feature was confined to the single genus, or was 

 general in the family, for only in rare instances would so ex- 



