92 Bulletin American Museum of Natural History. [Vol. XVI, 



I. THE ADAPTIVE RADIATION. 



The Oligocene Titanotheres consisted of at least four con- 

 temporary phyla, to which the prior generic names Titano- 

 therium, Megacerops, Syiiiborodon, and Brontotherium may be 

 applied. 



They represent an adaptive radiation for different local 

 habitat, different modes of feeding, fighting, locomotion, etc., 

 which took origin, in part at least, in the Middle or Upper 

 Eocene. Europe and Asia also may have shared in this 

 radiation, since Titanotheres are now definitely known in the 

 Balkan region. 



The main phyletic characters are analogous to those re- 

 cently (Osborn, 1900, p. 231) determined among Rhinoceroses; 

 the great antiquity of the lines leading to the existing species 

 of Rhinoceroses necessitated the revival of a number of dis- 

 carded generic names to distinguish them. Similarly the 

 separateness of four of the Titanothere phyla, throughout the 

 Oligocene and possibly from the Eocene, render it desirable 

 to revive certain generic names which in my first review I 

 considered undefinable. 



Radiation involved three main sets of characters, two of 

 which were correlated: 



First, dolichocephaly and brachycephaly, associated with 

 numerous changes in the skull and teeth, and, in at least two 

 phyla, with longer and shorter limbs. 



Second, four distinct types in the shape and position of 

 the horns, correlated with the structure of the nasals and 

 frontals and indicative of different modes of combat among 

 the males. 



Third, canines of different form; and, finally, the presence 

 of one or two pairs of functional incisor teeth, or the total 

 degeneration of these teeth. 



2. THE FOUR GENERA. 



Titanotherium Leidy applies to long-limbed animals with 

 long skulls, persistently long and broad nasals, short triangu- 



