28 



the merycoidodontim: 



Limnenetes, a contemporary of Oreonetes, has inflated bullae and was made a separate genus 

 largely on that basis, although originally Douglass placed both in the former genus while emphasiz- 

 ing the differences between the small and the large bulla;. 



On the basis of tooth and skeletal structures and of the variability of the size of the bullas in 

 the genus, I assume that Merycoidodon led directly into Eforeodon, that Merycoidodon was far 



Merycoides 

 lonqiceps 



Phenacocdelus 

 typus 



Ticholeptus 

 petersoni 



Metoreodon 

 relictus 



Fig. 5. — Third and fourth inferior and superior premolars of Merycoides, Phenacoccelus, Ticholeptus, and Metoreodon. 



(After Loomis, 1924.) 



Merychyus 

 minimus 



Leptauchenia 

 decora 



Fig. 6. — Third and fourth inferior and superior premolars of Merychyus and Leftaucheni-a. (After Loomis, 1924.) 



more widespread than we suppose at present, and that Oreonetes and Limnenetes should not be 

 separated on the ground of the presence or absence of inflated auditory bulla; alone. 



Our main difficulty lies in not knowing the complete skeletons of enough genera in this family 

 to make definite statements as to classification, so that the latter has to be made mainly on the char- 

 acters of the skull which, while very important, are not in themselves all sufficient. 



By early Miocene time we can clearly discern four phyla. These are respectively the Meso- 

 reodon-Pronomotherium, the Ticholeptus-Metoreodon, the Merychyus, and the Leptauchenia- 



