E. L. Troxell — Neiv Species of Hyracodon. 



39 



a way peculiar to Hyracodon alone. At a later stage of 

 wear, the two inner loplis unite by a narrow bridge. 



On the molars and large premolars there are developed 

 sharp cristas and crochets, a feature which is well known in 

 the totally unrelated later rhinoceroses, where the medi- 

 fossette in its extreme form becomes a lake. New char- 

 acters like these seem generally to arise on the outer 

 edges of the cusps, and then in the course of evolution 

 appear gradually more deep-seated. They are therefore 

 quickly worn away from the tooth of a more primitive 

 type. 



M-^ 



Fig. 4. — Hyracodon leidyanus, sp. nov. Holotype. Cat. No. 11169, 

 Y. P. M. X 3/4. In this large specimen the ridges of the premolars are 

 parallel and the median valleys uninterrupted. 



/^r Mr 



Fig. 5. — Lower molars of Hyracodon leidyanus, sp. nov. Holytype. X %• 



The antecrochet on M^-^ is very strong and a sharp 

 vertical groove divides it from the protocone. Two 

 grooves thus mark off the protocone, while one appears to 

 limit the hypocone. The cingular ridges do not cross the 

 bases of the inner cones of the molars as they do the 

 premolars. 



The lower cheek teeth, like the upper, are notable for 

 their general simplicity, subdued cingula, high crowns, 

 flat outer surface, and lack of angularity. In Hyracodon 

 there is not a great difference in the height of the anterior 

 and posterior lobes of the lower molars, as there is in 

 CcEMopus or even Trigonias. 



In the lower jaw, an alveolus reveals the former pres- 

 ence of Pi or Dp J, which as a permanent tooth is seen in 

 no other specimen of Hyracodon at hand, indicating in 

 general the advanced evolution of this genus. 



