1(38 THE WASATCH AND BRIDGER FAUN^. 



molar trenchant, and generally those anterior to it also, while in the Macro- 

 pidce the trenchant premolar, if present, is the third. The genera diflFer as 

 follows: 



a Several large cutting premolars. 



Premolars four, sides not riil;;ed Ctenacodon. 



I'reutohirs typically tliree, with oblique lateral ridges Plagiaulujc. 



a a One large cutting premolar. 



P Inferior molars large, with several tubercles. 

 Large premolar without posterior cusp; edge directed upwards; sides 



ridged Ptilodm. 



a a a Fourth premolar rudimental or wanting. 

 Large premolar with posterior cnsj); edge directed forwards; molars with 



two rows of tubercles Catopmlis. 



Fourth premolar? wanting; molars with three rows of tubercles Pohjmaxtodon. 



,? ,? Inferior molars small, with few lobes; the last rudimental. 

 Large premolar without posterior cusp; edge directed upwards; sides not 



ridged Thylavuko. 



Of the above genera, I'lagiaulax is represented by two species in the 

 English Jurassic; Ctenacodon by two species in the North American Jurassic; 

 Ftilodus by two species, from the Lower Eocene, one from France and one 

 from North America ; Thylacoleo by one species from the Pliocene of Austra- 

 lia; Catopsalis by two species from the Lower Eocene of North America; 

 and Polymastodon by one species from the Lower Eocene of North America. 

 The phylogeny of these forms in connection with that of the kanga- 

 roos may be expressed as follows: It is evident that such forms as 

 Thylacoleo^ Ptilodus, and Catopsalis are more specialized than Playimdax 

 ami Ctenacodon, inasmuch as the number of teeth is reduced, and the 

 cutting function of the premolars is concentrated in a single large tooth, 

 or is obsolete. This is quite the same kind of specialization as that 

 which has taken place in the history of the descent of the Carnivora. 

 Ctenacodon, as having the largest number of premolars, which have the 

 least amount of sculpture, is the least specialized of all the genera. 

 Thylacoleo, with the rudimental character of tiie true molar teeth, is the 

 most specialized, as it is the latest in time. The Alacropodida: retain 

 the full series of true molar teeth of the ])riniitive jMammalia, and pre- 

 sent only a cutting third premolar in the lower jaw, the fourth reseu)- 

 bling the true molars. Thus the cutting tooth of Thylacoleo is not the 

 homologue of the cutting tooth of Uypsiprymniis as suj)posed by Professor 

 Flower;* since tlu- latter corresponds with the cutting tooth of Ptilodus, 



' Quarterly Jonnml Gcologiciil Society, 1868, p. 307, vol.'zxiv. 



