\g2 Bulletin American Miiseum of Natural History. [Vol. XVI, 



Genus PELYCODUS Cope. 



Pelycodus is distinguished generically from the later mem- 

 bers of the family by the more tritubercular upper molars, 

 which exhibit the hypocone in all stages of development 

 (Fig. 20) and the quinquetubercular lower molars which 

 correspondingly show the paraconid in various stages of de- 

 generation (Fig. 22). P. jrugivorus is more tritubercular; 

 P. to/M5 is more sexitubercular; a constant distinction from 

 the contemporary Hyopsodus in the upper molars is the 

 presence of an internal cingulum in Pelycodus which is wanting 

 in Hyopsodus. In the Wind River specimens we also note 

 the rise of the external intermediate column or mesostyle in 

 a manner precisely analogous (homoplastic) to its develop- 

 ment in the equine Perissodactyla. In other words the 

 Lower Eocene Pelycodus is in a lower stage of evolution from 

 the tritubercular (tuberculo-sectorial) type than its successors 

 in the Middle Eocene (or Bridger) stage (Fig. 20). 



Dentition. — In examining the rich Cope collection, now in the 

 American Museum, the incisors are apparently |; the upper 

 pairs are conical and not spaced; the canines are slightly 

 enlarged and erect; the dental series is somewhat spaced, 

 that is, the first and second premolars are not crowded (Fig. 

 21). The lower molars show traces of a hypoconulid; the 

 first premolar is usually single fanged, and exceptionally bi- 

 fanged; the crowns of pms 1-2 are simple. The third and 

 fourth upper premolars show a single external cusp (proto- 

 cone) and an internal cusp (deuterocone) while the fourth 

 lower premolar is also slowly transforming into the molar 

 pattern by the addition of a tritoconid. The upper molars 

 may be clearly distinguished from those of the Microsyops line 

 by the stronger development of the intermediate tubercles or 

 conules (Fig. 20), which are exceptionally progressive, also 

 by the more rounded or quadrate contour. 



In the following descriptions dependence is placed largely 

 upon the specific determinations made by Cope himself. The 

 species undoubtedly require careful reexamination. 



Skeleton. — In the Wasatch species the jaw (P. tutus) is 



