206 



These teeth are sufficiently like the corresponding tooth in the jaw-speci- 

 men of Merychyus major to render it probable that Ibis animal may belong to 

 the same genus as the former. 



The last upper premolar of Merycoclimrus rusticus is like that of M. pro- 

 prius. 



The superior molars, the inferior premolars and molars, are so closely like 

 those of Merychyus elegans, that they may be considered as their magnified 

 representatives. 



Fig. 1, Plate VII, represents a series of upper molars in a specimen in 

 which the last one has not more than two-thirds protruded. A view of the 

 outer part of this last molar is introduced in the representation of the upper 

 jaw in Fig. 1, Plate III, so as to complete the series of upper molar teeth. 

 In the first molar the anterior crescentic enamel pit is observed to be com- 

 pletely obliterated, and the posterior one nearly so. In the back two molars 

 the inner faces of the internal lobes are decidedly concave longitudinally. 



In Fig. 6, Plate VII, we have a presentation of the first and second 

 upper molars of Merycochairus proprius introduced, for comparison. The 

 specimen is from the head-waters of the Niobrara River, in the vicinity of 

 Fort Laramie. 



Fig. 2, of the same plate, represents the last premolar and the molar of 

 the temporary series of M. rusticus. The molar is like those of the perma- 

 nent set ; the premolar resembles the former modified by having the anterior 

 lobes, especially the inner one, proportionately less well developed. 



In a small fragment of an upper jaw of another young animal, in which the 

 temporary molars were retained and the first permanent molar had protruded, 

 the maxillary presents a different appearance from that in the adult. The 

 surfaces above and below the position of the ridge produced by the malar 

 process are almost at a right angle to each other. The upper surface slopes 

 forward and outward from the position occupied by the orbit, and upon it 

 opens the infra-orbital foramen about half an inch within the ridge separat- 

 ing this surface from the lower one. In the progress of development from 

 youth to age the angularity of the outer part of the maxillary became rounded, 

 so that the surface assumed a convex instead of a nearly rectangular char- 

 acter. 



