124 ON THE EXTINCT MAMMALIA OF 



Notwithstanding the difTerences existing, and which I have attempted to indicate, 

 since I have had the opportunity of inspecting the additional specimens o( Lejjiauche- 

 nia above referred to, I have suspected that this genus may prove to be the same as 

 Merychyus. At any rate the latter is next of kin to the former, and represented it at 

 a later period. Oreodon, Merycochmrus and Leptauchenia were cotemporaries, their 

 I'emains being derived from the same stratum of the miocene formation, bed D of Dr. 

 Ilayden's section. The remains oi Merychyus belong to the pliocene formation of the 

 Niobrara, or bed F of Dr. Hayden's section. The latter is the pliocene Leplauchenia, 

 and was probably a direct ofispring of the miocene Leptauchenia. 



The fossil specimens referred to the latter apparently indicate three species, all of 

 which were comparatively small. 



Leptauchenia major. 



The largest species of Leptauchenia, distinguished by the above name, was originally 

 founded on specimens consisting of one side of an upper jaw, with all the molars and 

 the canine tooth, and several fragments of lower jaws with teeth. The last collection 

 of Dr. Hayden contains a nearly complete skull, which, however, is partially crushed, 

 much fractured and otherwise mutilated. 



The skull indicates an animal about the size of the smallest species of Merychyus, 

 and is intermediate in size to Oreodon Culbertsoni and 0. gracilis. In comparing the 

 specimens of jaws and teeth oi Merychyus elegaiis with the corresponding parts of the 

 skull oi Leptauchenia major, the resemblance appeared so great that I could not avoid 

 the suspicion that the discovery of additional material might prove them to be the 

 same, though I think it hardly probable that the differences which have been indi- 

 cated as of generic value can be due only to individual peculiarity. 



The specimen of Leptauchenia major, consisting of a portion of the upper jaw with 

 teeth, is represented in figure 4, plate XII. It belongs to the left side and is half an 

 inch shorter than the corresponding portion of Merychyus elegans. The bone is much 

 mutilated, but its form is observed to be nearly as in the latter. The infra-orbital 

 foramen is above the back part of the third premolar; in M. elegans it is above the 

 fourth. The alveolar border, with the teeth, is more convex, both downward and out- 

 wardly. It is alsa nearly uniformly convex in the former direction, while in M. 

 elegans it is sigmoid, or convex downward in the position of the true molars, and 

 concave in that of the premolars. 



The external buttresses of the true molars are strikingly prominent in comparison 

 with those of il£ elegans ; and from the greater obliquity of the intervening surfaces 

 of the outer lobes the buttresses have a more overlapping or imbricated appearance. 

 (Compare figures 4, 5, plate XII, with figures 1, 3, 4, plate XI.) 



The triturating surfaces of the same teeth, figure 5, present broad dentinal tracts. 



