126 THE MAMMALIA OF THE DEEP RIVER BEDS. 



in the disappearance of the median ridge on the outer side of the protocone, as well 

 as of the external cingulum, of which a trace is retained on M. The construction 

 of the internal side of the crown is very much like what is to be found in Oreodon, 

 but the ridges and hollows are, for the most part, better developed, and there are 

 other minor differences. In all the premolars the protocone is a compressed and 

 trenchant pyramidal cone, terminating below in an acute point which is in the median 

 line of the crown. The first premolar has a slightly convex external face and is not 

 so wide transversely as in the White River genus, especially in the posterior half; 

 on the front edge there is a fossa bounded by an internal ridge which descends par- 

 allel to the edge of the protocone, and a faintly marked posterior fossa is formed by 

 a slight elevation of the cingulum. This tooth is therefore of a somewhat different 

 shape and simpler pattern than p_i of Oreodon. The second premolar is larger than 

 pi , but has a similar external form with convex face ; internally there are two ante- 

 rior fossettes, the second of which is formed by the anterior cingulnm, and the pos- 

 terior fossette is much deeper than in p_2. All of these internal ridges are more 

 prominent than in the White River genus, but the transverse diameter of the crown 

 is less. The third premolar has a slightly concave external face, and the internal 

 crests and cingulum are better developed than in p_2. The anterior ridges are more 

 conspicuous than in Oreodon, but the posterior cingulum, or deuterocone, very much 

 less so, and this, combined with the narrower crown, gives the tooth quite a different 

 appearance in the two genera. In M we find the usual pair of crescents, the proto- 

 and deuterocones, as in the selenodonts generally ; the transverse width of the crown 

 is somewhat greater in proportion, the protocone more compressed laterally and the 

 valley narrower, though deep, than in Oreodon. 



The molars are like those of Eporeodon, but with a certain resemblance to those 

 of Merycliy us ; this likeness is to be seen in a heightening of the crown vertically, 

 narrowing of the valleys, the compression or thinning of the external buttresses 

 (para- and mcsostyles) and in the fore-and-aft extension of the postero-external 

 crescent in m. 3 . On the other hand, the characteristic feature of the Merycliyus 

 molar, viz., the extension of the posterior horn of the crescentic protocone, cutting 

 off the anterior horn of the hypocone from contact with the outer wall, is not present. 

 In m. 1 and m. 2 the postero-internal crescent is developed at the expense of the antero- 

 internal, which is especially small in m. 1, but in m. 3 the two are of nearly equal size 

 and the adjacent horns are in close contact at their extremities, m. 3 has a well- 

 developed outer fold or buttress at the hinder edge of the metaconid, which extends 

 beyond the posterior horn of the inner crescent as in JEjporeodon and Merycliyus, but 

 not in Oreodon. 



