UPPER CRETACEOUS MULTITUBERCULATA iii 



the two or three strong ridges which run outward on the slof)e of each cusp toward the 

 midgroove. Another type of Mi is seen in Y. P.M. No. 1 3681 , with the formuhi 6 : 4 and 

 measuring 2.3 mm. by i.o mm. The cusps resemble those of the tooth just described 

 save that they are more distinctly crescentic. The following data from a few of the 

 unbroken teeth in the Yale collection will give some conception of the variation and 

 show clearly that several species are present : 



Mo is exemplified by the type of Nanomys minutus Marsh, Y.P.M. No. 1 1855. It 

 measures 3.0 mm. in length and 2.2 mm. in width. Its cusp formula is best expressed as 

 4 : 2, although the posterior two cusps of the outer row are in contact almost to their tips 

 and posterior to the last one a ridge runs around the posterior end of the tooth which 

 probably bore several small cuspules when unworn. The two inner cusps are massive, 

 but they occupy only about two-thirds as much space as does the outer row. The ante- 

 rior border is squarely transverse, the posterior strongly oblique. Another tooth, almost 

 identical in character but unworn, shows the ridge around the posterior end of the 

 midgroove to have borne seven or eight minute tubercles. The central groove has the 

 usual pattern of irregular, sharp, narrow, anastomosing ridges. 



Quite a diflFerent tj-pe of Mo is seen in Y.P.M. No. 10399D. The cusp formula is 

 7 : 2. The outer cusps have their bases confluent, only the tips rising free, and from each 

 tip a strong ridge runs downward and backward into the midgroove. The posterior of 

 the two stout inner cusps sends five similar ridges into the midgroove. The other vari- 

 ants are less distinctive. The size range is rather less than for some other parts of the 

 dentition, probably because the smallest forms would be very difficult to find. 



Upper Incisors. — The upper incisors of this group are not surely known. The 

 large incisors with bifid tip were apparently confined to Menisco'essus, or at least no 

 teeth of this character in the collections appear to be small enough for Cimotomys. 

 Marsh assigned to this genus some teeth with three accessory cusps of decreasing size 

 posterior to the main point. The possibility that these are multituberculate incisors is 

 not flatly denied, but they are quite unlike those of any other member of the order. 

 They may be reptilian. Osborn (1893) states, "The median upper incisors are short 

 crowned and slightly notched at the back; they are closely applied with oblique pos- 



