io8 AMERICAN MESOZOIC MAMMALIA 



present, P4 relatively long and low with not less than eleven serrations. Molar cusps 

 crescentic, but with complex basal grooves and ridges. M2 longer than broad, with six 

 or more cusps. Posterior upper premolars elongate, with a main shearing series of 

 seven or eight cusps and a basal anteroexternal series of three or four. Jaw relatively 

 long and slender. Species smaller than those of Menisco'essus. 



Type. — Cimolomys gracilis Marsh. 



Distribution. — Lance formation, Wyoming; Hell Creek beds, Montana; Belly 

 River formation, Alberta. 



As here used, this genus probably includes two or three genera, possibly even 

 more. By Osborn all of these Cretaceous smaller multituberculates were referred to 

 the Paleocene (Torrejon, Fort Union) genus Ptilodus. That Ptilodus should occur 

 in the Cretaceous is very improbable, a friori, and now proves to be demonstrably in- 

 correct. The differences between multituberculate genera are always more clearly seen 

 in the last upper premolar than in any other single tooth. I n Ptilodus this tooth has 

 two complete elongate longitudinal rows of cusps of about equal development and the 

 rudiments of a third row at the anteroexternal edge of the tooth. From the Lance a 

 number of homologous teeth are known which must be adequately representative of 

 Cimolomys, even in its broadest sense and these always have only one complete row of 

 cusps with the rudiments (or vestiges) of a second row anteroexternally. The two 

 genera cannot be considered synonymous. The lower jaws, to be sure, are very similar, 

 but the lower jaw pattern in the whole family is rather stereotyped. Species can be dis- 

 tinguished from lower jaws alone, but in many cases the generic characters are here 

 rather obscure. 



Included Genera 



1889. Cimolodon, Marsh, Anier. Jour. Set. (3) XXXVIII, 84. Type: C. nitidus Marsh. 



1889. Nanomys, Marsh, Amer. Jour. Set. (3) XXXVIII, 85. Type: N. minutus Marsh. Non 

 Nannomys Peters 1876. 



1892. Nanomyop, Marsh, Amer. Jour. Set. (3) XLIII, 261. New name for Nanomys. 



1889. Allacodon, Marsh, Amer. Jour. Sci. (3) XXXVIII, 178. Type: ^. lentus Marsh. Some of 

 the material referred to this genus probably should be included in Menisco'essus but the 

 genoholotype probably belongs with the genera here included under Cimolomys. 



Cimolomys gracilis Marsh 1889 

 1889. C. gracilis. Marsh, Amer. Jour. Sci. (3) XXXVIII, 84. 



Type.— Y.P.M. No. 11835. Isolated M\ 



Horizon and Locality. — Lance, Niobrara County, Wyoming. 



Diagnosis. — M^ type 4.8 mm. long and 2.7 mm. wide. Cusp formula 6 : 8 : 9. Inner 

 row as long as outer. Species used in a very broad sense for all of the smaller multi- 

 tuberculates from the Niobrara County, Wyoming, Lance beds. 



