E. L. Troxell—CcEnopus, the Ancestral Rhinoceros. i3 



In general, the genus is distinguished by the presence 

 of two upper incisors, the absence of the canines (see 

 Leptaceratherium below), the presence of rudimentary 

 horn cores or none at all, parallel lophs on P^ and the 

 tendency toward this arrangement in other premolars in 

 later species. 



The generic group is composed of C. trigonodus 

 (Osborn and Wortman), and C. copei (Osborn), two 

 species which seem to lead respectively to C. platyceph- 

 alus, the terminal member of its line, and to C. tridactylus, 

 which probably furnished the source of all later forms, 

 certain possible immigrants excepted. 



Ccenopus {Leptaceratherium) trigonodus (Osborn and 

 Wortman) as a type possesses the upper canine and thus 

 forms the connecting link between this genus and Tri- 

 gonias. This species has a further important primitive 

 feature, a loop uniting the cross lophs of the premolars 

 through the deuterocone and tetartocone, which are so 

 blended as to appear as one element ; in this respect also 

 it approaches T. oshorni, while on the other hand it trends 

 toward C. platycephalus, its probable successor. 



A new subspecies of C. trigonodus is described on a 

 later page in this paper. It illustrates an advanced step 

 in the evolution : has already lost P and C\ and has devel- 

 oped a very prominent deuterocone on P^ which envelops 

 the thin sinuous metaloph (Cat. No. 12052, Y. P. M.). 



Ccenopus platycephalus (Osborn and Wortman) is typ- 

 ically an Upper Oligocene species, but smaller and more 

 primitive specimens have been reported. Its type is 

 marked by a broad low cranium and nearly obsolete sag- 

 ittal crest ; the species is also noted for the distinct sepa- 

 ration of the tetartocone from the metaloph, the great 

 reduction of the latter, the simplicity of the molars, and 

 generally for the large size of the individuals. 



A new Subspecies will be proposed (Cat. No. 12489, Y. 

 P. M.) which, because of the extreme reduction of the 

 metaloph and the expansion of the deuterocone to occupy 

 the whole inner portion of the larger premolars, is 

 thought to be the termination of its line, and to be in no 

 way related to later genera. 



Ccenopus copei (Osborn), although it is barely distin- 

 guishable from C. occidentalis (Leidy) (heautotype, the 

 holotype being lost), except in size, and although it resem- 

 bles very much the paratype maxillary of C. mitis (Cope) , 



