260 EOCENE FAUNA. 



/3/3. Third superior premolar without internal cusps; anterior cusps of inferior 

 molars present. 



Cusps of superior molars marginal ; two siijjerior incisors . . . Lepticliis Leidy. 



Cusi)s of sufierior molars median in position; anterior cusps of inferior molars well 



developed ; anjrle of mandible not inflected Peratherium Aym. 



ftfifi. Anterior cusi)s of inferior molars wanting. 



Fourth inferior i>remolar like true molars Uiacodun Cope. 



II. Fourth inferior premolar ditlerent from true molars in a simpler constitution. 

 a. Inferior true molars not (or not all) tubercular sectorial. 

 Last inferior molar tubercular; cusi)s of other true molars well develoiied; three pre- 

 molars above and below Deltatheriiim Cope. 



Inferior true molars alike, with anterior inner cnsps little developed; three premo- 

 lars (?) TriisodoH Cope. 



aa. Inferior true molars all tubercular sectorial. 

 Inferior true molars alike, with cusps well developed ; four compressed premolars below, 



three above Didelphodus Cope. 



Premolars four below, robust, conic Quercitherium Filh. 



Premolars four above and below, compressed ; the fourth superior with a conic cusp 



and heel externally Styolo2)hus Cope. 



Premolars four below, compressed; fourth superior with a simple blade externallj. 



Proviverra Riitim. 

 MlACID^. 



Inferior tubercular molars two, premolars four Miacis Cope. 



Inferior tubercular molars one, premolars lour Bidymictis Cope. 



OXYJENIDiE. 



I. Inferior molars without internal tubeicles. 



Molars, ^ f ; three sectorials in the lower jaw Pterodon Blv. 



II. Inferior molars with internal cusjis. 



a. Posterior heel of one or more superior molars elongate and trenchant. 

 Last inferior molar truly sectorial, without internal tubercle; second tubercular-sec- 



torial Protopxalis Cope. 



Molars, a 5; two last inferior molars tubercular sectorial Oxyana Cope. 



Amblyctonid^e. 

 Fourth inferior premolar with a broad heel supporting tubercles; an anterior and no 



internal tubercles A7nblyctoiiii>i Cope. 



iJental formula below, 3, 1, 3, 3. Fourth inferior premolar with a cutting edge on the 



heel ; both internal and anterior tubercles Palaonyctis Blv. 



^Iesonychid^. 



a. Inferior molars seven. 



Cones of inferior and superior molars simple. Mcsonyx Cope. 



Cones of last two inferior molars with lateral cusps Dixsacus Cope. 



an. Inferior molars f six. 

 Internal lobes of penultimate superior molar v-sbaped Sarcothrausles Cope. 



ari'i. Inferior molars five. 

 Inferior molars with strong anterior lobe. tPatrio/clis Leidy. 



