FORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 



101 



different, (d) only P4 can be compared, and (e) extremely slight differ- 

 ences in this tooth are commonly associated with specific or even 

 generic differences in other parts of the dentition/" 



In size this species is a little closer to E. musculus than is E. grangeri, 

 although still definitely larger, but its other distinctions from E. 

 grangeri are distinctive in about the same degree from E. musculus. 



A tooth of this species (no. 6088) was referred by Gidley to "Ptilodus 

 serratus" {—Halodon serratus Marsh) in his preliminary publication 

 (1909, p. 622) but with the reservation that it might later be placed 

 in a new species. Had he completed his work. Dr. Gidley would 

 unquestionably have placed this specimen in a new species, as is done 

 here. The remarks made above, regarding specimens of fPtilodus 

 sinclairi referred tentatively to "Ptilodus formosus" are equally 

 apropos here. 



Table 20. — Measurements of individual specimens of ?Ectypodus russelli 



7ECTYPODUS SI.LBERLINGI Simpson 



Figure 10, c 

 f Edypodus silberlingi Simpson, 1935d, p. 226. 



Type. — U.S.N.M. no. 9798, left lower jaw with incisor and P4-M2. 

 Collected by A. C. Silberling. 



Horizon and locality. — Gidley Quarry, Fort Union, Middle Paleocene 

 horizon. Crazy Mountain Field, Mont. 



Diagnosis. — (Only one specimen.) Length P4, 3.3. Length Mi, 

 2.3. Ratio length P4:length M„ 1.4. Length Miiwddth Mi, 2.6. 

 Serrations P4, 12. Cusps Mi, 9-10:5-6."^ Crest of P4 relatively low. 



Remarks. — In most of its characters, this species falls weU within 

 the range of fPtilodus sinclairi, with which it would be confused in 

 casual examination, but its deviation from the mean of sinclairi in 

 length of Ml is 5 times the standard deviation of the latter, and in 

 the ratio length Mi '.width Mi, 2.3 times, and the marked difference 

 in cusp number of Mi is also surely significant. The species cannot 

 be, and the genera probably are not, the same. 



<' Russell (1929. p. 173) in describing E. cochranensis was doubtless referring to my IE. rusnlli when he 

 wrote, "Undescribed premolars of about the same size as E. cochranensis occur in the Fort Union beds of 

 Montana, but these teeth differ markedly from the present specimen in having a prouounced undercutting 

 in the anterior margin of the crown." 



*» That is, clearly at least 9:5, and in each row rudiments of another cusp so that it is doubtful whether 

 the count should be 9:5 or 10:6. 



