I04 



AMERICAN MESOZOIC MAMMALIA 





is 



"^■F^ \\. 



/nside 



Superior 

 /afen»r 



A 



Oulside 



/naide 



Outside 



B 



(ts 



/nside 



Ouiaide 



lower incisors of this genus. Its association is confirmed by the fact that it fits the alveo- 

 lus of a good specimen ( Y.P.M. No. 10613) with much of the lower jaw intact, includ- 

 ing P3-4 and Ml. The crown is pointed and saber-like, the median surface flat where it 

 was appressed against its fellow, the outer surface convex. The enamel covers the entire 



crown but ends abruptly at the mouth of 

 the alveolus. It is marked by a number of 

 grooves and ridges radiating from the 

 tip. The root tapers to a point beneath the 

 anterior end of Mi and contains a large 

 pulp cavity which ends anteriorly at 

 about the beginning of the true crown. 



Lower Premolars. — The specimen 

 mentioned above, Y.P.M. No. 10613, 

 contains both lower premolars. They are 

 preceded by a wide diastema. P3 is much 

 reduced, one-rooted, pillar-like, its tip 

 inserted in a notch beneath the overhang- 

 ing anterior part of P4. It is very similar 

 to that of Ptilodus. P4, in this jaw, is of 

 the shearing type so well known from the 

 Paleocene members of the family. The 

 cutting edge is semicircular and had ap- 

 parently nine serrations, although these 

 cannot be counted posteriorly. There are 

 eight oblique ridges on the lateral faces 

 of the crown. Y.P.M. No. 11810 {Halo- 

 don scnlftus) is similar to the above but 

 is slightly shorter relative to its height 

 and has coarser striations. It may repre- 

 sent a distinct species. Comparable teeth 

 are common in the collections and gen- 

 erally show nine projections on the shear- 

 ing edge and seven or eight distinct lateral ridges, the posterior ones rather faint. One 

 specimen, at least, appears to have only eight serrations. The teeth may at once be dis- 

 tinguished from those referred to Cimolomys by their larger size, shorter and higher 

 proportions, and relatively small number of serrations. 



Lower Molars. — Y.P.M. No. 11825 {Difriodon lunatns) is Marsh's type Mi. 

 As is commonly true in multituberculates, it is narrower than any referred Mo, meas- 

 uring 4.1 mm. in width and 8.7 mm. in length. The cusp formula is 5:4." The struc- 

 ture is quite simple for a ptilodontid. Each cusp is strongly crescentic, the crescents 



^' That is, five cusps in the outer row, four in the inner. The outer row is mentioned first, the 

 middle row, if present, next, and the inner row last in giving cusp formulae for multituberculates. 



Inside 



w 



Outside 



Fig. 41. Multituberculates. Lower incisors. A, C^e- 

 nacodon. B, Meniscoessus. C, Cimolomys. D, Taenio- 

 labis. B is completely enamel covered. The enamel is 

 solid black in the other drawings. A transverse sec- 

 tion of B is given. Not to scale. 



