1898.] IVortman, Extinct Camelidce of North America. lOI 



molar, which is considerably narrower and has a less development 

 of the internal cusp. Another important distinction between the 

 two species is found in the greater development of the vestigial 

 hypocone of the second superior molar of B. elega/ts. In 

 /)'. montauies this cusp has almost entirely disappeared, the only 

 evidence of its presence being indicated by a cingulum in this 

 portion of the crown. 



One fact of great interest in connection with this genus is the 

 probable light which it throws upon the homologies of certain 

 cusps of the molar crown in the higher selenodont Artiodactyla. 

 It is here that we witness the actual passage from the bunodont 

 to the selenodont type of molar in this important group. If we 

 can trust the evidence before us, Bunomeryx is a direct lineal 

 descendant of the Bridger Homacodon, and it is a matter of the 

 utmost moment to note that in the latter genus there are six fully- 

 developed cusps upon the crowns of the first and second superior 

 true molars ; in the third there are only five cusps present. In 

 Bunomeryx, as already indicated, the full six cusps are found on 

 the first superior molar only, while in the second molar there is 

 'but a vestige of the postero-internal cusp or hypocone. The 

 evidence appears to be conclusive, therefore, that the true homo- 

 logical hy[)ocone is in process of retrogressive disappearance, and 

 in proportion as this cusp is reduced, the posterior intermediate 

 is pushed out to take its place. As a further evidence of the 

 truth of this proposition it may be stated that the true hypocone 

 of both the first and second molars of Ho/nacodon, as well as the 

 first molar of Bunomeryx, exhibits no tendency whatever to 

 develop a selenodont structure, while the posterior intermediates 

 especially in Bunomeryx, exhibit very decided advances in this 

 direction. The very position of this cusp, moreover, precludes 

 any possibility of its entering into the formation of the single 

 posterior internal crescent of the more perfectly developed seleno- 

 dont molar of the higher types. 



I believe therefore that the history of the formation of the four 

 crescents of the superior molar crowns of the Selenodonts has 

 been as follows, tracing it from the five-cusped Pantolestes^ of the 



' From this genus I exclude the type of Pantolesies ctsagicus Cope as belonging to a distinct 

 genus ancestral to and leading directly up to the bunodont Artiodactyla. It is very probably 

 synonymous with Eohyus distans of Marsh, who properly placed it among the Bunodonts. 



