1902.] Osborn, American Eocene Prvnates. 181 



narrow form and pointed cusps of the lower molars, both upon 

 talonid and trigonid, readily distinguish the members of this, 

 family from the Notharctidag. All the known species in- 

 clude animals of small size. 



Hyopsodus does not show a progressive increase in size; 

 for example, the largest Wasatch species, H. powellianus, 

 (Fig. 9) is larger than the largest known Bridger species. 



Premolar and molar transformation. — Bridger species of 

 Ungulates, and in fact of all bunodont mammals, are generally 

 more complex in dentition than Wind River species, and 

 invariably far more complex than Wasatch species, so we 

 cannot agree with Cope in identifying Wasatch specimens 

 of Hyopsodus with typical Bridger species such as H. panlus. 

 The species therefore require thorough rearrangement accord- 

 ing to geological succession and the law of progressive com- 

 plication of the molar and premolar teeth. 



The grinding teeth gradually become more complex, homo- 

 plastic with those of Ungulates. The upper molars progress 

 from a triangular, tritubercular condition with a rudimentary 

 hypocone to a quadrate, sexitubercular condition with a 

 prominent hypocone (Bridger and Uinta); the external cin- 

 gulum increases in strength until it entirely extends across- 

 the outer surface of the crown (H. marshi) ; it does not 

 develop a mesostyle. The upper premolars progress by the 

 addition of internal cusps; the fourth upper premolar has 

 an internal cusp (deuterocone) in the Wasatch specimens ; 

 the fourth, third, and second have internal cusps in the upper 

 Bridger specimens. The third and fourth lower premolars 

 are similarly transformed, but less rapidly. 



If we should follow the same principle as that which obtains 

 among the horses, the species of Hyopsodus in successive 

 geological stages might well be separated as genera but it 

 would not subserve clearness to do this. 



I. Wasatch (Sparnacien, Ypresien) Stage. 



Common characters of the Wasatch species. — Superior molars 

 sub-triangular with hypocone depressed and rudimentary or 



