32 E. L. Troxell — Amynodonts in Marsh Collection, 



Most of the enamel is broken away from M^. It is 

 slightly smaller than M2, which is similar in size and shape 

 to Mo. The last is but little worn and shows well the two 

 crescentic ridges so characteristic of the rhinoceroses. 



ff4^3 TYPE Y. P. M. 



Fig. 6. — Amynodon erectus, 

 teeth. X 1/3. 



sp. iiov. Holotype. Crown view of lower 



Summary of A. eredus, sp. nov. — The holotype is based 

 on a very well preserved skull and jaws, more primitive 

 than A. iniermedius, but more advanced than A. (Ortlio- 

 cynodon) antiquiis. It is smaller than any of the other 

 species. It presents evidence of erect canines both above 

 and below, and in this respect resembles A. antiquiis; 

 from this species it differs, however, in its later geological 

 age, smaller size, and especially in the absence of both 

 upper and lower first premolars. 



The holotype of A. advenus Marsh, Cat. No. 11763, 

 Y. P. M., consists of a single third upper molar. A. erec- 

 tus, sp. nov., may be distinguished from it by the stronger 

 cingula both fore and aft, the deeper postsinus, the much 

 narrower medisinus with a basal cingulum and cusp 

 obstructing its opening, and finally, its smaller size. 



a 



Fig. 7. — a, Amynodon advenus Marsh. Holotype. Cat. No. 11763, 

 Y. P. M. l, A. erectus, sp. nov. Holotype. Cat No. 11453, Y. P. M. 

 Third upper molar of each species. Note the differences of size ajid 

 form : the variation in the anterior and posterior cingulum, median val- 

 ley, and internal basal cusp, and the ectoloph extended backward. Nat. size. 



