ORDINAL TYJ'KS OF MOLARS: RODKXTIA 



147 



FIG. 108. Facial portion of skull, and lower jaw of M/ilngaulus monoi/on, from the Upper 

 Miocene of Colorado, a Sciuromorph Rodent (family Mylagaulidse), in which the true molars are 

 reduced and the fourth premolar greatly enlarged (Cf. Fig. 107). x J . From Matthew. 



Mylagaulus, 

 Middle Miocene. 



Meniscomys, 

 Upper Oligocene. 



Lower Miocene 

 (After Sinclair). 



Hdplodontia 

 (the Sewellel or Moun- 

 tain Beaver). Recent. 



ml. 



' pr. 



FIG. 1011. Traces of trituberculy in the fourth upper premolars of the Rodent families 

 Mylagaulidte and Haplodontiidas. " Osborn's molar cusp nomenclature is used as a matter of 

 convenience, not necessarily implying homology with the cusps of the true molars." (Matthew.) 

 If the fourth upper premolar in these families has been derived from a tooth with the trituber- 

 eular/om, the first molar was probably tritubercular inform and in origin. After Matthew. 



FIG. 110. Upper check teeth of E-utypomys tliomsoni, an Oligocene Castorid Rodent from 

 the Oreodon Beds, allied to the Beavers, but with less hypselodont and more primitive teeth, 

 which apparently retain traces of the protocone, hypocone, paracone, metacone, and of the 

 external styles (pas, >nx, mix), x-3.. After Matthew. 



