40 E. L. Troxell — New Species of Hyracodon. 



The external vertical groove of the lower teeth curves 

 backward in a way nnlike that of any other specimen 

 observed, and, especially in the two larger premolars, 

 continues into the cingular ridge instead of ending 

 abruptly against it. 



Summary. 



There are three families of extinct rhinoceros -like 

 animals : the true Rhinocerotidse, gi^T.ng rise to our 

 modern animals ; the Amynodontidse, culminating in 

 Metamynodon in early Oligocene time ; and finally, the 

 Hyracodontidse, represented by the single genus Hyraco- 

 don, found in the Lower and Middle Oligocene of the 

 Great Plains. This genus is easily distinguished by its 

 small size, its slender proportions, and the presence of all 

 canine and incisor teeth. The trend of its evolution 

 seems to have been toward the loss of the lateral toes, a 

 cursorial adaptation. 



Two new species arid one subspecies have been estab- 

 lished here in order to set forth features which either have 

 been ill defined, or are entirely new in the genus. 



Measurements of Holotypes. 



R., H. E. 

 mimus leidyanus selenidens 



Upper jaw : mm. mm. mm. 



Premolar series, length 67 74 57 



Molar premolar series, length 129* 142* 110 



P\ length 13 14 11 



Width 14 16 12 



P-, length 16 18 14 



AVidth 19 21 16 



ps, length 18 21 15 



Width 23 25 19 



P^ length 20 22 16 



Width 26 27 21 



M\ length 24 19 



Widtli 25 21 



M^, length 27 20 



Width 27 21 



M^ length 17 



AVidth 21 



Lower jaw : 



Length of three premolars 61 46 



Length of three premolars and M, ... 85 63 



Length of molar premolar series 101 



*Estima.ted. 



