386 THE EOCENE PERIOD. 



TI. Tiitorinediate tubercles wanting; inferior premolars without internal lobes. 



Superior molars with post4?rior internal cusp only, besides internal V; last two supe- 

 rior premolars with internal lobes Anisoiichiis. 



Superior molars with internal V only, no other Internal lobes; last two superior pre- 

 molars with internal cusps Hemithltnts. 



Superior molars with posterior internal cnsp only, besides apex of V; fourth superior 

 premolar only with internal lobe Haploconxis. 



PHENACODONTID.fi. 



The genera of this family display a uniformity in the structure of the 

 true molar teeth not seen in the PeriptychicUB. Their range of grade is seen 

 in the premolars, especially those of the superior series. Thus in Protogonia, 

 all of those teeth have but a single external lobe. In Phenacodus, the 

 fourth has two external lobes. In Dlacodexis, the second, third, and fourth 

 teeth have two external lobes. The premolars are unknown in Anacodon. 

 While Protogonia is primitive in its superior premolars, its inferior true 

 molars come nearer to developing V's than any other genus of the family. 

 The definitions are as follows: 



Last superior premolars with but one external cusp; inferior molars with Vs. 



Protogonia. 

 Last superior premolars with two external cusps; inferior molars with well-developed 



cusps Phenacodus. 



Inferior molars with flat frrinding faces; no cusps Anacodon. 



Second, third, and fourth superior premolars with two external cusps ; those of inferior 



molars well developed I>iacodexi«. 



Meniscotheriid.*:. 



This family includes the single genus Meniscotherium. 



Superior molars with intermediate tubercles, the anterior crescentic, the posterior 

 oblique, forming a crest with the posterior inner; anterior inner conic. Inferior 

 molars and last premolar with two Vs; other inferior j^remolars without int< rnal 

 lobes; fourth superior premolar with two external lobes Meniscotherium. 



