112 AMERICAN MESOZOIC MAMMALIA 



terior faces, in which the dentine is exposed. ..." Teeth of this sort are not uncommon, 

 but they may be P of Meniscoessus. Some forms of Cimolomys may have resembled 

 Ptilodus in having such simple upper incisors that they could hardly be recognized as 

 such in individual specimens. 



Upper Premolars. — The last upper premolar of this group is readily identified 

 and was correctly associated by both Marsh and Osborn. Y.P.M. No. 10375 (Marsh 

 1892A, PI. VI, fig. 2) is typical. It belongs with one of the smaller forms, measuring 

 only 3.0 mm. by 1.4 mm. The main feature is a long sharp crest which bears seven 

 cuspules, the sixth the largest. Posterointernally the tooth slopes out into a small heel 

 which may have borne a cusp now removed by wear. External to the first four cusps of 

 the shearing crest is a parallel series of four cusps. These are low on the crown, almost 

 basal in character, but individually they are larger than those of the main crest. In 

 another small type (Y.P.M. No. 13718), 4.9 mm. in length, the posterointernal heel is 

 less developed, there are only three cusps in the anteroexternal accessory row, and pos- 

 terior to the main crest is a pair of small basal cuspules opposite each other trans- 

 versely. Twenty-five or thirty teeth of this general type have been examined. They 

 show about the same range of size as do the other teeth included in this group. 



The anterior upper teeth of this group are seen in Marsh's ALlacodon. A. lentus 

 has five conical cusps arranged in two transverse pairs and with the fifth at one end of 

 the tooth in a median position. A. fumilis has four cusps arranged in a square, one pair 

 stouter than the other, and A. rarus is of essentially the same type. A. fords has three 

 subequal cusps in a triangle. Now that the upper dentition of Ptilodus is well known, 

 it is clear that these different types owe their characters more to position in the series 

 than to systematic differences. I n Ctenacodon there are three tricuspid premolars fol- 

 lowed by two multicuspid shearing teeth. In Ptilodus there are in series a tricuspid 

 tooth, a quadricuspid one, and a quadri- to sexcuspid one, followed by a single multi- 

 cuspid shearing tooth. In U.S.N.M. No. 2138 (incorrectly said to be the type of A. 

 fumilis in the Catalogue of tj^es, etc., in the National Museum) the three anterior 

 upper premolars of a member of this group are preserved in actual association (Marsh 

 1892A, PI. VII, fig. 3). The first two teeth are tricuspid, the third quadricuspid. These 

 were probably followed by the shearing tooth, although it is possible that another tooth 

 with four to six cusps intervened. The latter would be contrary to analogy with Ptilo- 

 dus, however, and it is more likely that the teeth in the collections with five and six 

 cusps belonged to different species and occupied the position of the quadricuspid tooth 

 in this series. 



Upper Molars. — Cimolomys was originally based on a first upper molar, Y.P.M. 

 No. 1 1835. It measures 4.8 mm. by 2.7 mm. and has a cusp formula of 6:8:9. The 

 inner row is slightly narrower anteriorly, but it is as long as the other two rows, a very 

 unusual character among Ptilodontidae. Cimolomys bellus was also based on a first 

 upper molar, Y.P.M. No. 1 1778. It is 3.0 mm. long and 1.6 wide and has a formula of 

 6:7:7. The last cusp of the outer row is small and all those of the inner row are small. 

 The inner row narrows anteriorly and quite disappears opposite the posterior end of 

 the second cusp of the median row. C. digona, Y.P.M. No. 1 1779 is larger, measuring 



