protocone 



protocone 



tritocone 



tetartocone 



deuierocone' 



tetactocone 



FIG. 180. Progressive molarization of the preruulars from pi - />4 in Hyrttchyu^agrarius, family 

 Hyracodoutidre, a Rhiuocerotoid from the Bridger Formation, Middle Eocene 



A 



Flo. 181. Four stages in the evolution of the molars in the Rhinoceroses. After Gaudry. 



A. Aco-ntherium Umaneiise, Upper Oligocene, Europe. 



B. Rhinoceros pachygnathus, Lower Pliocene, Europe. 



C. Rhinoceros antiquitatis, Pleistocene, Europe. 



D. Elaxinotfierium sibericum, Pleistocene, Europe. After Gaudry and Boule. 



We note (1) the progression from brachyodonty to hypselodonty, ('2) from roots to open-pulps, 

 <3) the increasing verticality of the slopes of the crests, (4) the development of the cement, (5) the 

 increasing plication of the enamel. Stage A parallels the Stegodonts among Proboscidea 

 (pp. 180-188). Stage D parallels the elephants, and also many of the South American Ungulates, 

 and even the rootless grinders of many Rodents. 



protocone 



-ff metalopMd 



h)lpolof)h i ri 



FIG. 182. Extreme specialization of the Rhinoceros molar type, in Elasmotherium sibericum, 

 from the Pleistocene of Siberia. The plicatiun of the enamel has mere IM-.I the rutting surface, 

 and the efficiency of the tooth for grinding hard dry grasses or shrubs. (Compare the enanu-l 

 lakes in the Horse molar, Fig. 103). x +. After Gaudry and Boule. 



