DAKOTA AND NEBRASKA. 133 



The alveolar border accommodating the molar teeth is less convex antero-posteri- 

 orly than in OreoJoii. 



The portions of the lower jaw, accompanying the specimen just described, corres- 

 ponding with the position of the molar teeth, resemble the same portions in Orcodon. 



The number of teeth possessed by Ar/riochcenis cannot be ascertained from the 

 specimens; it may, however, be suspected, from the evident affinity of the genus to 

 Oreodon and its allies, that it had an equal number. 



In one of the specimens (figure 6, plate i, Anc. Fauna of Neb.) there are preserved 

 on one side of the upjier jaw a series of six molar teeth, the true molars and 

 three premolars in advance. In the accompanying portions of the lower jaw five 

 molars are retained, — the true molars and the adjacent two premolars. 



The true molars o? Agrhchoerus are evidently of the ruminant type, though very 

 peculiar compared with those of recent ruminants or other extinct members of the 

 order. 



The upper true molars are remarkable for the wide-spreading character of 

 their crowns. They bear a striking resemblance to those of the extinct Ilijopotamus 

 in form and proportions, except that they do not possess the odd or fifth lobe situated 

 between the anterior pair of usual lobes. In comparison with those of Oreodon they 

 are proportionately of less depth and greater breadth. The constituent lobes have 

 nearly the same form and relative proportions, but are lower, more expanded, and 

 separated by wider and shallower interspaces. Their external buttresses are of the 

 robust hemispherical character of those oi Hyopotamus, and are not laterally com- 

 pressed in descending from the base as in Oreodon. 



The upper last premolar resembles the teeth behind, with the postero-internal lobe 

 reduced to a rudimentary condition, and with the external buttresses of the crown 

 also much reduced in their proportions. 



The penultimate premolar has a three-sided pyramidal crown, with a compara- 

 tively small three-sided lobe situated postero-internally. The principal lobe is 

 considerably larger than those of the succeeding tooth, and has its broader external 

 surface rather convex transversely and provided with three slight vertical ridges at 

 its back half. 



The antepenultimate premolar is a reduced form of the succeeding one. 



The lower true molars nearly resemble those of Oreodon in form and proportions, 

 but their constituent lobes are rather smaller, more contracted towai'ds the summit, 

 and their interspaces are wider or more excavated in appearance. The inner sides of 

 the crowns resemble those of Oreodon or of the Deer, except that the lobes are less 

 oblique, and do not overlap one another where contiguous. The posterior lobe also 

 is provided with a recurved buttress-like process contiguous to that of the anterior 

 lobe. 



