1881.] 1^* [Cope. 



Last superior molar longitudinal ; inferior true molars without developed 



sectorial blade Amblyctonidm. 



II. Ankle-joint tongued and grooved, or trochlear. 

 Molar teeth in both jaws consisting of conic tubercles and heels ; none 



sectorial ; a preglenoid crest Mesonychidm. 



I now give the charactei's of the genera. All these are derived from 

 examination of tj'pical specimens. The opportunity of doing tliis 1 owe 

 to the kindness of Messrs. Leidy, Gervais, Gaudry, Filhol, and Lemoine. 



Arctocyonidye. 



Premolars, f ; the first inferior one-rooted ; the last inferior well developed ; 



Arctocyon Blv. 

 Premolars below, 4, the first two-rooted, the last true molar much reduced ; 



(fide Lemoine) . ; Eyodectes Cope. 



Premolars below, 3, first two-rooted ; true molars normal 



Heteroborus Cope. 



MlACID^. 



Inferior tubercular molars two, premolars four Miaci» Cope. 



Inferior tubercular molars one, premolars four \Didymktu Cope. 



Leptictid^e. 

 I. Superior molars sub-equilateral, without cutting heel posteriorly. 

 a. Fourth inferior true molar like the true molars, with three anterior 



cusps. 

 ^. Third superior premolar with internal cusp ; anterior cusp of in- 

 ferior molars small, median. 

 Third premolar with one external and one internal cvi^YiS.Mesodectes Cope. 

 Third premolar with two external and one internal cusps. . . .Ictops Leidy. 

 /?;?. Third superior premolar without internal cusps ; anterior cusps of 

 inferior molars present. 

 Cusps of superior molars marginal ; two superior incisors ; Leptictis Leidy. 

 Cusps of superior molars median in position ; anterior cusp of inferior 



molars well developed Perathermm Aym. 



y?/?/?. Anterior cusps of inferior molars wanting. 



Fourth inferior premolar like true molars Biacodon Cope. 



aa. Fourth inferior premolar different from true molars in a simpler 

 constitution. 

 Last inferior molar tubercular ; cusps of other true molars well developed; 



three inferior premolars Lipodectes Cope. 



Inferior true molars alike, with anterior inner cusps little developed ; three 



premolars (?) Triisodon Cope. 



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



Deltatherium Cope. 



