no AMERICAN MESOZOIC MAMMALIA 



Dentition 



Lower Incisors. — There is a distinctive type of lower incisor which, as Osborn 

 has pointed out, must from its invariably small size belong to the group here united 

 under Cimolomys. Marsh also recognized the true affinities of these teeth and made one 

 (Y.P.M. No. "i-ijyy) the type of his species Cimolodon -parvus. It is slender, pointed, 

 the median side concave, the outer convex. The tip is completely enameled. All of the 

 extra-alveolar part of the tooth is covered with at least a light coat of enamel except for 

 the posterosuperior part, toward the inner face. The enamel extends a short distance 

 into the alveolus along the lower side. It is impossible to affirm positively that the 

 larger forms did not have incisors more like those of Meniscoessus, but in general the 

 two seem very distinct, with no intermediates. There are, however, a number of pos- 

 sible variants and doubtful teeth and no multituberculate tooth is so easily confused 

 with that of a reptile or of some other mammals. 



Lower Premolars.— Y.P.M. No. 13692 has P4, Mi, and Ma in association and 

 hence puts the characters of the lower cheek teeth in the genus beyond much question. 

 The premolar is badly worn, but permits close comparison with perfect isolated speci- 

 mens. The following comparisons are instructive : 



Length P4 Length P1.4 



Species Length Mi Length M^ 



Ctenacodon serratus 1.31 30° 



Meniscoessus sp. 0.92 



Cimolomys sp. i.6o 



Ptilodus mediaevus 2.30 ... 



Ctenacodon probably represents approximately the structurally ancestral condi- 

 tion. Ptilodus has about the same relative shearing power, but it is concentrated in a 

 single tooth rather than three (P2-3, Pi being hardly shearing in Ctenacodon). Cimolo- 

 mys has a relatively much shorter shearing blade than Ptilodus, although it was still 

 very important and although the opposing upper tooth was even more perfectly adapted 

 for this purpose. Meniscoessus shows the greatest reduction in relative length of the 

 blade although, as has been shown, it is here still well developed and shows no loss of 

 function. 



The size of P4 varies about as much as does that of the molars (below), but its 

 proportions remain about the same. The number of serrations varies from eleven in the 

 smaller to fourteen in the larger specimens. 



P3 has eluded the collectors, but its alveolus is known and it was obviously like 

 that of Meniscoessus or Ptilodus. 



Lower Molars. — The type of Cimolodon nitidus (Y.P.M. No. 11776) is a first 

 lower molar. It measures 4.5 mm. in length by 2.0 mm. in width and has a cusp for- 

 mula of 8 : 4. The outer cusps, of which the first two are small, are crescentic but less 

 strongly so than in Difriodon. The tips appear as if they had been bent forcibly back- 

 ward over the anterior slopes of the succeeding cusps. The inner cusps are larger, point 

 somewhat backward, and are faintly crescentic, although this is somewhat obscured by 



