UPPER CRETACEOUS MULTITUBERCULATA 107 



The larger of the anterior premolar teeth in the Yale collection were carefully 

 measured and their ratios to the lengths of M' and AP calculated for Menisco'cssus and 

 Cimolontys in an effort to determine whether they might belong to the larger genus. 

 The figures established for these genera and the comparative ratios for earlier and 

 later allies need not be repeated here, but the results may be briefly put in words. The 

 figures show that even the largest anterior upper premolars in the collections would be 

 smaller relative to the molars of Menisco'essus than are these teeth in Ptilodus or in 

 Ctenacodon. On the other hand they are considerably too large to have belonged to 

 even the largest species of Cimolontys, and they are no smaller relative to the molars 

 of Meitiscoessus than is the probable last upper premolar of this genus. The results are 

 not conclusive, but they point with considerable probability to the presence in Menis- 

 co'essus of upper premolars in addition to the last one. These were reduced in size, and 

 possibly also in number, and their true arrangement is entirely unknown. 



Upper Molars. — Selenacodon fragilis is based on M^ of Menisco'essus, but the 

 type (Y.P.M. No. 11897) consists only of the four fragments of an incomplete tooth 

 cemented together. Its proportions and some of its other characters have been altered. 

 Y.P.M. No. 10632, however, is a homologous tooth of this genus and all its characters 

 may be checked on several other excellent specimens. The cusp formula is 7 : 8 : 6, but 

 the posteroexternal, posterointernal and anteromedian cusps are smaller than the 

 others. The outer and median rows are of about equal length and width. In the inner 

 row the third to fifth cusps from the anterior end are the largest, and the row narrows 

 anteriorly and does not extend as far forward as the other two. The cusps are crescentic 

 in general form, the limbs of the crescents pointing forward. 



In orienting these teeth Osborn naturally assumed the shorter, tapering cusp row 

 to be external (1893, PI. VII, fig. 8, also in Cimolomys {"Ptilodus"] fig. 4), but this 

 is surely incorrect. In those multituberculates (save the Tritylodontidae) in which 

 there are three upper molar cusp rows, the incomplete or tapering row is always inter- 

 nal, its widest end posterior, on M^ and external, its widest end anterior, on Ml 

 Osborn's left M^ is from the right side and v'lce versa. 



M' of this genus was named Tr'ipr'wdon coelatus by Marsh. The type, Y.P.M. No. 

 1 1853, is 6.6 mm. long and 6.4 mm. wide, being much more nearly equidimensional 

 than in the specimens from South Dakota and Montana. The cusp formula is 3 : 3 : 4- 

 The cusps of the median row are crescentic, although, as in M\ modified by basal 

 grooving. The cusps of both lateral rows are rounded on the side away from the mid- 

 line but each sends a sharp crest toward the latter, curving also somewhat forward. 

 The outer cusp row narrows posteriorly and is shorter than the other two. 



Other teeth homologous with this differ in being somewhat larger, with small 

 cuspules at the anterior ends of inner and outer rows and several minute cuspules con- 

 tinuing the outer row posteriorly. There may also be four distinct median cusps. 



C/wo/owyj Marsh 1889 

 1889. Cimolomys, Marsh, Amer. Jour. Set. (3) XXXVIII, 84. 



Definition. — Lower incisor slender, not strongly compressed, enamel smooth. Ps 



