138 Annals of the Carnegie Museum. 



be regarded as the Bridger representative of this line of cursorial 

 Rhinoceroses.^^^ The Washakie genus Triplopus and the Uinta form 

 Prothyracodon uintense^^^ obviously represent independent lines, 

 which may or may not be represented in the Oligocene,"^ while P . 

 obliquidens is in all the obtainable characters so very closely related 

 to the Oligocene genus Hyracodon that one cannot deny the phylo- 

 genetic relationship here displayed. It is very unfortunate that we 

 have not in the recently acquired collection from the Uinta a skull 

 sufficiently complete in the region of the tympanum and the external 

 ear to verify Professor Cope's studies of Triplopus cubitalis. Con- 

 sidering all the known characters of Prothyracodon which are so very 

 suggestive of Hyracodon, I believe that the Uinta form did not have 

 the meatus closed inferiorly as in Triplopus cubitalis. This would 

 substantiate Cope's position in placing Triplopus in a separate syste- 

 matic position. Together with this equine feature of the external 

 ear in Triplopus we now know that the limbs were also proportionally 

 longer than in the Uinta genus. The genus apparently does, in fact, 

 represent a subfamily {Triplopodincc) of the Hyracodontidce which 

 holds an equal rank to the Prothyracodon-Hyracodon phylum. 



Together with the highly Rhinocerotic feature of M- in Prothyra- 

 codon uintense the first upper and lower premolars are altogether too 

 much reduced in size^^'' to be seriously regarded as a forerunner of the 

 rhinoceroses of the Oligocene. Furthermore the lower canine and 

 incisor series are typically those of Hyracodon, plainly excluding this 

 species from the true rhinoceroses of the later Tertiary. In my opinion 

 it is altogether possible, that, if this line continued in later epochs, we 

 may find a Hyracodon-like form in the Oligocene with M- reduced to 

 the characteristic features of the Rhinocerotidcc so strongly suggested 

 in the Uinta genus. 



1'' The actual type of Hyrachyus implicalus Cope I have not seen, but from the 

 splendid illustrations (Tertiary Vertebrata, PI. LVIII, Figs. 6, 6a, 7) by Professor 

 ■Cope, it appears to be in this line. Its dentition seems to be advanced in the direc- 

 tion of the Hyracodonts. 



134 \Yith the exception of M', Prothyracodon uintense bears a closer relation to 

 Hyracodon nebrascencis than does Triplopus cubitalis. 



1^5 The different forms ol Hyracodon of the lower and upper Oligocene are as yet 

 comparatively little known. 



'28 Both the upper and lower first premolars may well be absent in fully adult 

 specimens of P. uintense as they are, in fact, seen to be in individuals in the Car- 

 negie Museum. 



