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



The anterior premolars increase rapidly in size and P- 

 is a third wider than P\ P- is snbquadrate, having three 

 sides at right angles. On this tooth the protoloph exhib- 

 its the prominence which in part characterizes the species. 

 Its form, however, is not that of a perfect crescent, for it 

 shows an irregular cnrve and a ridge or pillar where it 

 joins the tetartocone. The metaloph is a thin straight 

 w^all dividing the equal-sized medi- and postfossettes. 



P^ resembles P^ in nearly every respect, but shows a 

 greater decrease in the size of the metaloph and a 

 smoother crescent. On P^ the deuterocone is set off by 

 vertical grooves running up the sides ; this cone shows a 

 strong tendency to lean toward and to occupy the central 

 portion of the tooth. The prominence of the base of the 

 cone gives the tooth a somewhat triangular outline. 



There is a small crista on P^^ and a small crochet on the 

 metaloph of P^ near the tetartocone, partly separating a 

 portion of the medifossette or valley. There is evidence 

 of a faint crista on M-, but the crochet seen on other 

 specimens is undeveloped. The antecrochet is not so 

 conspicuous as in H. leidyanus, Cat. No. 11169. 



The molars are smooth on their inner cones, and in gen- 

 eral show extreme simplicity. The crowns are relatively 

 low. 



Hyracodon leidyanus, sp. nov. 



(Figs. 4-5.) 



Holotype, Cat. No. 11169, paratype, Cat. No. 11168, Y. P. M. Middle or 

 Lower Qligocene, Crow Buttes, South Dakota. 



The holotype of this species consists of a maxillary and 

 ramus with tooth series P^ to ]\P and P2 to Mj, inclusive. 

 The paratA^e material includes two specimens ; one may 

 be a part of the holotj^e but that can not be demonstrated. 

 The parts preserved are: second upper molar, atlas, 

 vertebra, numerous toe bones, tibia, astragalus, navicular, 

 metatarsus, and broken parts of the calcaneum, radius, 

 metacarpal III, another tibia, and a second metatarsus. 



The holotype is unusual in its large size, smooth teeth, 

 molariform premolars, and high crowns, together with 

 the following additional features: The protoloph of P^ 

 joins more intimately with the metaloph than with the 

 ectoloph and is very much decreased in size. The meta- 

 loph of P-, which is only submolarif orm, extends at right 

 angles from the ectoloph and at its end hooks backward in 



