E. L. Troxell — New Species of Eyracodon. 37 



surrounding all teeth; deuterocone very prominent on 

 P^ and set off by vertical grooves ; this tooth is subtrian- 

 gular. None of the premolars can be said to be molari- 

 form. 



Hyracodon selenidens, sp. nov. 



(Figs. 2-3.) 

 Holotype, Cat. No. 11173, Y. P. M. Middle Oligocene, Colorado. 



This new species of Hyracodon is especially notable 

 for its small size and the crescentic form of the deutero- 

 cone, hence the specific name. The holotype is about 

 three fourths the size of B. leidyanus described later, and 

 is therefore the smallest of the Oligocene rhinoceroses. 

 The species possesses certain features typical of Ccenopus 

 of an entirely different family, indicating parallel or con- 

 vergent evolution ; the complete enveloping of the meta- 

 loph by the protoloph, and the diminution of the former 

 are seen in C. alius and reach an extreme in C. nanolo- 

 phus, ncAv forms described in a later paper of this series. 



Fig. 2. — Upper cheek teeth of Hyracodon selenidens, sp. nov. Holotype. 

 Cat. No. 11173, Y. P. M. X 3/4. The smaller cross crest is completely 

 encircled by the other on the premolars of this very small species. 



Fig. 3. — Lower molars and premolars of the small Hyracodon selenidens, 

 sp. nov. Holotype. X 3/4. 



P^ departs from the pattern of the other premolars; 

 its protoloph is scarcely more prominent than the cingu- 

 lum posterior and is separated from the inner main cone • 

 the metaloph bisects the tooth and ends in a cross on its 

 outer end. The tooth forms roughly the half of an ellipse. 



