FORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 



85 



The circumstance that the two previously named species of Fort 

 Union ptilodonts are synonymous makes of no interest the question 

 whether Osborn's incorrect reference of Cimolomys gracilis Marsh to 

 Ptilodus invahdates Gidley's later Ptilodus gracilis and robs Hay's 

 name Ptilodus admirabilis of any raison d'etre. 



The distinction of this species from the very closely related Ptilodus 

 mediaevus of the Torrejon has already been discussed in part as exem- 

 plifying the methods used in this research. The more important 

 comparisons may be summed up as follows: 



Figure S.— Ptilodus montanus Douglass, U.S.N.M. no. 6076, left lower jaw (with some details completed 

 from right lower jaw of same individual): a. External view; 6, crown view. Three times^natural size. 



Most dimensions not significantly different, but in the one specimen 

 of P. mediaevus that has this tooth the deviation of the width of P^ 

 from the mean in P. montanus is 2.7 times the standard deviation of 

 the latter. 



Serrations of P4, mode 14 in P. montanus, and of six specimens of 

 P. mediaevus five have 12 and one 13. 



Cusps of P^, mode 6 in P. montanus, and of three specimens of P. 

 mediaevus two have 4 and one 5. 



External cusps of P*, mode in P. montanus, and one specimen of 

 P. mediaevus has two. The development of this external shelf is 

 stronger in this specimen of mediaevus (and in another in which the 

 cusps cannot be surely counted) than in any specimen of montanus. 



