182 



EVOLUTION OF MAMMALIAN MOLAR TEETH 



FIG. 178. Superior molar pattern of a primitive Hyracodont Rhinoceros Hvrachyus agrarius 

 from the Bridger Formation, Middle Eocene, showing a general resemblance to the Lophiodont 

 type, from which this differs in the smaller paracone and flatter metacone. The crista as in 

 lihinoceroses generally is merely a portion of the internal face of the paracone (cf. Fig. 175). 



frtnloph 



taloplt 



protolopli 



arlocone 



jirot 



D 



FIG. 179. Molarization of the fourth upper premolar in Ccnwpvs occidcntalis, a Rhinoceros 

 from the Oreodon Beds, Middle Oligocene as shown in four specimens from successive geological 

 levels. In A, the protoloph is imperfect, the metaloph irregular, in D the protoloph is complete, 

 the metaloph fairly well developed. 6 and C are more or less intermediate. 





