CEEODONTA. 259 



gradation between the triangular Didelphi/s-Mke and the sub-sectoi'ial Ptero- 

 don-\\ke forms of the superior molars in this group of genera. 



The definitions of the families will then be as follows : 



I. Ankle-joint plane transversely, or nearly so. 



True molars above and below, tubercular; last superior not transverse .. Arctoei/owid«p. 



Superior true molars, tubercular ; last superior premolar sectorial ; first inferior " tuber- 

 cular sectorial" MiacidcB. 



Superior molars triangular, last one transverse; inferior molars tubercular-sectorial, 

 or with reduced anterior cusp ; no preglenoid crest Leptictidce. 



Last superior molar trenchant, transverse ; first superior true molar sectorial ; inferior 

 true molars tubercular-sectorial ; a preglenoid crest Oxycenidce. 



Last superior molar longitudinal; inferior true molars without developed sectorial 

 blade Amhlyctonidce. 



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



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

 preglenoid crest . Mesonychidce. 



I novp give the characters of the genera. All these are derived from 



examination of typical specimens. The opportunity of doing this I owe to 



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



Arctocyonid^. 



a. Superior molars with two internal tubercles. 

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



Arctocyon Blv. 

 Premolars |; first two below one-rooted; superior molars with two internal cusps. 



Achcenodon Cope. 

 aa. Superior molars unknown. 

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

 moine) - Hyodectes* Cope. 



Premolars below, 3; first two-rooted ; true molars normal Heteroboriis\ Cope. 



aaa. Superior molars with one internal tubercle. 

 Premolars below, 4, without internal tubercles; first one-rooted ; superior molars with 

 an internal V, and intermediate tubercles Mioclmmis Cope. 



Leptictid^4 



I. Fourth inferior true molar like the true molars, or with three anterior cusps. 



/?. Third superior premolar with internal cusp; anterior cusp of inferior molars 

 small, median. 



Third premolar with one external and one internal cusps Mesodecfes Cope. 



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



* Type Arctocyon gervaimi Lemoine, Oss. Fobs, des Envir. de Reims, 1878, p. 8. 

 tType Arctocyon duelii Lemoine, loc. cit., p. 9. 



tThe genera referred here resemble considertibly tlie family Didelphidce. The species of LepUclU 

 and Mesodectea have thus far only been found in the White River beds; see Part Second of this volume. 



