38 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I28 



the conclusion that Achaenodon and Archaeotherium, though both 

 possibly somewhat similarly adapted as large bunodont, piglike ani- 

 mals, in many details of dental structure followed divergent lines. 

 Achaenodon is almost surely derived from the Bridger Helohyus, and 

 Parahyus is an admirable link between the two. There appear to be 

 no important obstacles to such an interpretation, and the resemblance 

 in the dentition is striking. Achaenodon achieved considerable size 

 during the interval between Bridger D and Uinta B (or Washakie B) 

 time and developed a somewhat shortened snout, apparently with the 

 loss of a premolar, but the remaining premolars appear to have become 

 robust, though closely placed or crowded. The increased size of the 

 premolars is foreshadowed in Helohyus lower-jaw material from 

 Bridger D referred to H. lentus. In details of the teeth the Achaeno- 

 don premolars retain their simple carnivorelike form, and there is 

 little change in the lower molars except size and a somewhat more 

 inflated appearance of the cusps. In the upper molars, however, there 

 has been a trend toward simplification. The rather weak hypocone, 

 which is little more than a prominence of the cingulum in Helohyus, 

 is apparently lost in Achaenodon, and the protoconule has been much 

 reduced or is absent so that the upper molars have become essentially 

 four cusped. 



Archaeotherium, which is first known in the Oligocene, may have 

 evolved from Helohyus or quite possibly Lophiohyus, but this is un- 

 certain. In contrast to Achaenodon, Archaeotherium developed an 

 elongate snout and retained all its premolars in a well-spaced arrange- 

 ment. It further developed various bony protuberances on the jaw 

 and arch, and the postorbital processes of the parietal and jugal 

 joined. In details of the teeth, P3, rather than P4, above and below, 

 became the more prominent or highest crowned of the premolars. 

 The upper molars, rather than becoming more simplified in their 

 structures, gave increasing prominence to the hypocone and proto- 

 conule so that the Archaeotherium upper molar has essentially six 

 nearly equal cusps and has further given rather marked prominence 

 to the anterior and posterior cingula. Retention of cusps in the lower 

 molars is noted in the rather distinct paraconid and prominent hypo- 

 conulid, except that in M3 the hypoconulid is strikingly reduced (for 

 this tooth) from the primitive structure exemplified in Helohyus 

 and in even greater contrast to the development of this cusp in 

 Achaenodon and Parahyus. 



