DAKOTA AND NEBRASKA. 117 



The lower true molars of Merychyus are not only longer and slightly narrower 

 relatively than in Oreodon, but are remarkable for the compai-ative shallowness of the 

 interspaces separating the inner and outer constituent lobes. Indeed, before the last 

 molar is fully protruded, the interspaces of its lobes are obliterated, as well as those of 

 the molars in advance. Before the teeth are half worn away, the lower true molars 

 ajDjiear composed of an antero-posterior pair of three-sided prisms, with the addition 

 of the usual back lobe to the last of the series. (See figures 5, 6, 14, plate XI.) 



The inner surface of the crowns of the lower true molars is a nearly vertical plane, 

 as In the Camel, interrupted only by feeble ridges at the division of the lobes. In 

 Oreodon the corresponding surface presents a similar appearance to that in the Deer. 

 (Compare figures 7, 13, plate XI, of the j^resent work, with figure 3, plate iii, and 

 figure 9, plate vi, of The Ancient Fauna of Nebraska.) The outer surfaces of the 

 crowns resemble those in Oreodon, except in their greater relative length, more ver- 

 tical character, and in appearing more concave longitudinally. 



The crowns of the premolars of Merychyus are also longer and narrower in propor- 

 tion to their breadth than in Oreodon. 



The fourth upper premolar is nearly like that of Oreodon. Its outer face, besides 

 being lai'ger, is devoid of the slight median ridge, and is more convex longitudinally. 

 The interspace separating its lobes does not become so contracted as in the true 

 molars, its inner surface gradually shelving towards the bottom as in Oreodon. 



The anterior three upper jjremolars are constructed on the same plan as those of 

 Oreodon. The first one is smaller in its relation with those behind than in the 

 latter, and it is obliquely inserted across the alveolar border. 



Viewed externally, the anterior three upper premolars appear relatively larger 

 than in Oreodon, and less cordiform in appearance. They also exhibit a backward 

 curve not evident in Oreodon. They are further convex downward and transversely. 

 Their points are in advance of the middle, arising in part from a narrowing of the 

 front portion of the crown in relation with the back portion, and in the case of the 

 anterior two premolars, from the edge of the crown in advance having an inward 

 direction. In Oreodon the points of the premolars are median. (Compare figures 1, 

 15, plate XI, with figures 1, 2, plate VI, and figure 1, j^late VIII.) 



The internal portion of the crowns of the anterior three upper premolars present 

 the same ridges, sub-lobes and fosste as in Oreodon. 



The lower premolars are longer and slightly narrower in relation with their 

 breadth than in Oreodon, but otherwise closely resemble those of the latter. The 

 anterior pair, as in MerycocJicerus, are obliquely inserted across the alveolar border, 

 and more crowded in position than in Oreodon. (Compare figures 2, 5 — 8, plate XI, 

 with figures 1, 2, plate VI.) 



The canines and incisors, in form, position and relative size, closely resemble those 



