116 ON THE EXTINCT MAMMALIA OF 



in proportion to their breadth, and are also rather narrower in proportion to their 

 diameter antero-posteriorly, especially in the case of the last of the series. 



In Oreodon the crowns of the true molars protruded so as to be inserted alone by 

 the fangs at a comparatively early period, as in the Deer. In Merycliyvs they 

 successively protruded to the same extent at a much later period, as in the Camel. 



When the crown of the last upper molar was yet involved one-third in the jaw, in 

 Merychyus, the usual interspaces separating the inner and outer lobes of the first 

 molar and the anterior interspace of the second molar had become obliterated. See 

 figure 4, plate XI. In Oreodon the corresj)onding interspaces were retained long 

 after the complete protrusion of the crovras of all the molars. 



In Merychyus the outer and inner constituent lobes of the upper true molars are 

 closer, and the crescentic interspaces more contracted than in Oreodon. The ex- 

 ternal faces of the outer lobes are larger, less sloping, and more uniformly concave 

 in the transverse direction. They are slightly convex longitudinally, and generally 

 undivided by a median ridge. The internal surfaces of the inner lobes are more 

 concave vertically. 



In the unworn ujaper true molars of Merychyus the crescentic interspaces or pits 

 are widely gaping, but rapidly narrow to a vertical fissure. The external face of 

 the inner lobes, bounding the pits internally, is convex dovsmward and concave 

 transversely. The internal face of the outer lobes, bounding the pits externally, is 

 nearly vertical. In Oreodon the corresponding interspaces are more expanded, less 

 deep, and gTadually slope to the bottom. The external face of the inner lobes, 

 bounding the pits internally, is concave longitudinally and transversely, and the 

 internal face of the outer lobes, bounding the pits externally, is somewhat sloping. 

 (Compare figures 3, 4, 16, plate XI, with figure 1, plate VII.) 



The buttresses bounding the outer lobes externally of the upper true molars of 

 Merychyus are much longer than, but do not expand in the same robust manner at 

 bottom as in Oreodon. (Compare figures 1, 15, plate XI, with figures 1, 2, plate VI, 

 and figure 1, plate VIII.) 



The posterior horn of the antero-intemal lobe enters the angular interval of the 

 outer lobes, and is joined just before its termination by the contiguous horn of 

 the postero-internal lobe. In Oreodon the anterior horn of the latter lobe enters 

 the angular interval of the outer lobes, and the contiguous horn of the antero-internal 

 lobe is bent parallel with the former horn towards the postero-internal face of the 

 antero-external lobe. (Compare figure 16, plate XI, of the present work, with figure 

 1, plate iii, of The Ancient Fauna of Nebraska.)* 



*The difference is not well represented in the figures of the present work, in consequence of inattention on 

 the part of the artist. 



