FORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 137 



Type oj Megopterna minuta. — Carnegie Mus. no. 1675, left lower 

 jaw with M2 and part of talonid of Mi. Collected by A. C. Silberling. 



Horizon and locality. — Types from Silberling Quarry, most referred 

 specimens from Gidley Quarry, Fort Union, Middle Paleocene horizon, 

 Crazy Mountain Field, Mont. 



Diagnosis. — Sole known species of genus. Dimensions given below. 



Remarks. — From alveoli it is clear that this species had a much 

 enlarged, procumbent anterior tooth, probably an incisor, with a 

 compressed root. U.S.N.M. no. 9866 includes Mi of P. silberlingi 

 and also a loose tooth, which probably is an associated lower incisor. 

 It has a completely enameled crown, curving sharply to a point. The 

 whole crown has a series of eight or nine ridges or angulations, diverg- 

 ing posteriorly from the apex, so that in transverse section it is 

 irregularly polygonal. Aside from these, one side is more convex, the 

 other somewhat excavated, ^vith a slight basal cingulum. 



Figure 2i.—Picrodus silberlingi Douglass, U.S.N.M. no. 9622, right lower jaw: a, Crown view; 6, external 



view. Four times natural size. 



The anterior"tooth is followed by three small, closely spaced, ap- 

 proximately equal alveoli. The material does not indicate whether 

 these were for three separate teeth or for one 1-rooted tooth and 

 one 2-rooted. The next tooth, presumably P4, has two roots in the 

 several specimens that show it, not one as stated by Douglass for the 

 type. It has a simple main cusp, more procumbent than shown in 

 Douglass' figure, followed by a small heel. The next tooth, pre- 

 sumably Ml, is the largest in the jaw and is very peculiar. It differs 

 considerably from Douglass' figure, although I believe that the pres- 

 ent specimens do belong to his species and that the discrepancy is 

 due to the worn and broken nature of his specimen and the impos- 

 sibility of accurate observation except under a binocular microscope 

 at magnifications of 15 or 20 X. This tooth consists of a trigonid and 

 talonid, but both are greatly modified. The trigonid is small and is 

 produced and procumbent, as if drawn forward and upward in a 

 plastic condition. The protoconid lies near the midhne of the tooth 

 as a whole. The metaconid is slightly lower, poorly separated from 

 the protoconid, and internal and sHghtly posterior to the latter. The 



