36 E. L. Troxell — Neiv Species of Hyracodon. 



The published measurements show this species to be a 

 half larger than "H. nehrascensis" ; it even surpasses 

 the large H. leidyanus, sp. nov. 



f Hyracodon planiceps Scott and Osborn. 



This is a very large rhinoceroid which, by the authors,^ 

 is doubtfully referred to Hyracodon. 



Description of New Species. 

 Hyracodon arcidens mimus, subsp. nov. 



(Fig. 1.) 

 Holotype, Cat. Xo. 11174, Y. P. M. Oligoceiie, Deadwood, South Dakota. 



The holotype consists of both maxillaries with all 

 premolars in excellent preservation. It is evident that 

 this is near the species Leidy first described in 1850, 

 because the measurements are close ; it corresponds in 

 turn to the specimen figured in plate XII A, 1852^ and 



Fig. 1. — Upper premolars of Hyracodon arcidens mimus, subsp. nov. Holo- 

 type. Cat. No. 1117-i, Y. P. M. X 3/J:. Xote especially the continuous 

 internal loops, the cristae, and the prominent deuterocone of P*. 



resembles the maxillary shown in plate XIV, figs. 4-6, 

 1854.*^ Cope 's description of H. arcidens shows it to have 

 the same sort of looping protoloph, but his slightly larger 

 holotype is not figured, so no close identification is pos- 

 sible. 



A summary of the distinctive features is : strong crista 

 on the larger premolars ; protoloph joins the tetartocone 

 and on P^-"^ it completely encloses the thin straight meta- 

 loph ; deuterocone and tetartocone united in P^ but with 

 a deep double groove on the outside ; cingula completely 



* Scott and Osborn, op. cit., p. 171. 



^Joseph Leidy, in Owen's ''Keport of a geological survey of Wisconsin, 

 Iowa, and Minnesota, ' ' etc. 



® Joseph Leidy, Smithson. Cont. KnowL, vol. 6, art. 7. 



