Cimolomyidae

C. gracilis Marsh, 1889 b Meniscoessus major (Russell, 1937) M. robustus (Marsh, 1889 b) M. conquistus Cope, 1882 M. borealis Simpson, 1927 a

Cimolomys clarki, new species

The Cimolomyidae comprise medium- to large-sized multituberculates, with a tendency to increase in size in later forms. P 4 is a high, arcuate tooth, with fewer serrations than in most contemporary allotherians. The posterior two or three serrations of P 4 s in both genera are more distinct than the other serrations and may be deflected posteriorly; on the labial sides, grooves are present on or between these serrations and lead ventrally to a prominent posterolabial ledge containing one to three cuspules. P 4 differs in the two genera. It is long and low in Cimolomys, but in Meniscoessus it is short and high. The internal row of Ml is well developed, terminating at or near the second cusp from the anterior end of the medial row. The species of these genera from the Judith River Formation are more similar to each other than they are to later species.

The family was proposed by Marsh (1 889 c) but never defined. Clemens (1 963 b) preferred to place the genus in the Ptilodontidae, incertae sedis. Material from the Judith River Formation, however, indicates that the genera Cimolomys and Meniscoessus are closely related and should be included in the family Cimolomyidae. The affinity of the two genera as cimolomyids had already been anticipated by Sloan and Van Valen (1965) in their discussion of the Hell Creek multituberculates. The family as here defined contains two genera, Cimolomys and Meniscoessus, and the following six species: