616 PROCEEDINGS OF THE NATIONAL MUSEZTM. vol. xxxvi. 



Type-locality. — Sweet Grass County, Montana. 



Horizon — Fort Union formation. 



The description as given by Douglass is as follows: 



Crown of pi semi-elliptical in a lateral view; crown not high, and upper por- 

 tion of cutting edge not extremely convex ; eleven distinct and two posterior 

 indistinct ridges on the crown ; nu nearly one-balf the length of P4, with four 

 external <^ and six internal " tubercles ; anterior portion of tooth narrower than 

 posterior portion. 



The last premolar is larger than that of Ptilodua troucsaurtiunus Cope, but 

 not so large as that of P. mcdiavus Cope. 



Mcafiurcmcnt.s of tmic (after Douglass). 



mm. 



Length of pi 7.5 [7. S] » 



Length of uu " 3.7 [3.4] 



This is the first species of Ptilodus described from the Montana 

 Fort Union and is apparently quite distinct from the New Mexico 

 Torre j on species described by Cope. In the National Museum collec- 

 tion there are portions of upper and lower jaws representing four in- 

 dividuals that are probably referable to this species. They are two 

 left lower-jaw fragments, each containing p^; a lower-jaw fragment 

 containing p^ and m^; and an upper-jaw fragment containing the 

 three anterior premolars. Those are, respectively, Nos. GOTT, ()0T8, 

 ()()Ti), and GOSO, U. S. National Museum collection. They are from 

 the same locality and horizon as the type of P. niontanus. 



PTILODUS GRACILIS, new species. 



Type-ftpecimen. — A partial skeleton, including a nearly complete 

 skull and lower jaws, the greater portion of the pelvis, one fenuir, the 

 distal portion of the humerus, a radius and portion of an ulna, the 

 proximal end of a tibia, a terminal phalanx, portions of vertebrae, 

 and other fragments. (Cat. No. GOTO, U.S.N.M.) 



Type-locality. — Sweet Grass County, Montana. 



Horizon. — Fort Union formation.'^ 



General definition. — Dental formula, i|, cj, puif , m|. In size this 

 species is about intermediate between P. mediwvus and P. troaessar- 

 tianus. It is but slightly smaller than P. montani/s Douglass. The 

 lower jaw is far more slender in proportions than either P. mcdicpvus 

 or P. montanus ; the fourth premolar is relatively higher and has 14 

 ridges on either side of the crown. Lower m^ has but 5 tubercles in 



"This is evidently a typographical error and should read "four internal and 

 six external tubercles." 



^ In carefully remeasuring the type I find these measurements to be : Length 

 of p4, 7.8 mm. ; length of mi, 3.4 mm. 



'^ Other forms from these beds indicate that this horizon is about equivalent 

 to, as reported by Douglass, or perhaps somewhat older thau the Torrejon 

 beds of New Mexico. 



