UPPER CRETACEOUS MULTITUBERCULATA 105 



opening backward, and all are of about the same size save the reduced anteroexternal 

 cusp. They are not arranged in pairs transversely. 



Several variants, perhaps representing as many species, are known. Thus in 

 Y.P.M. No. 10608 the small anteroexternal cusp is almost vestigial and there is a simi- 

 lar minute cusp at the posterior end of the same row. In Y.P.M. No. 106 14 the cusps 

 are more nearly opposite transversely. 



M2 is well seen in Y.P.M. No. 11234, the type of the species.'* This specimen 

 measures j.-j mm. in length and 5.5 mm. in width and has a cusp formula of 4: 2. The 

 first three of the external cusps are subequal, the last a little smaller. The outer slope 

 of each external cusp is conical and the inner slope is produced into a sharp crest curv- 

 ing inward and backward. The anterior of the two inner cusps is crescentic, opening 

 backward, and is larger than any of the outer cusps. The posterointernal cusp is of 

 about the same size and is more nearly conical, although two crests run backward and 

 a little outward on its external slope. The longitudinal midgroove is marked by antero- 

 posterior motion of the opposing row of cusps. It is deep and V-shaped. The bases of all 

 the cusps are marked by small rugations in the midgroove. The crown ends squarely 

 in front, but the posterior border is at an angle of about 45° to the long axis, the outer 

 row being longer than the inner. The coronoid process arises external to this tooth and 

 the alveolar border ends abruptly posterior to it. 



Upper Incisors. — It is impossible to say whether V was present. It was already 

 very small in the Jurassic. P was certainly correctly identified by Osborn and had also 

 been correctly associated by Marsh with his Halodon, which is included in this genus. 

 The median surface is slightly concave, the outer broadly convex. The tip is bifid, the 

 two sharp points being so arranged as to make the tip somewhat spatulate, or with an 

 approach to a transverse chisel-edge, but this is not produced nor accentuated by wear 

 against the lower incisors, which are sharply pointed. About halfway up the high 

 crown on its posterior side is another sharp cusp. The extra-alveolar part of the tooth 

 is completely covered with a thick coat of enamel, which does not extend down into the 

 alveolus. In life the tooth was apparently very slightly procumbent. 



I' is apparently represented by similar teeth, but they are smaller, with lower 

 crowns, the tip single. A posterior accessory cusp is present as on V. 



Upper Premolars. — Speaking of the upper premolars of Meniscoessus, Osborn 

 says (1893, p. 319), "No teeth are found which can with certainty be considered the 

 upper premolars of Meniscoessus; we naturally look for something similar to the upper 

 premolars of Ptilodus, and we find it represented most nearly in the types Oracodon 

 ancefs and O. conulus Marsh." This the present writer considers correct, with not 

 much greater uncertainty than attends the whole process of reconstructing an animal 

 by patchwork. There are about a dozen good specimens similar to Marsh's Oracodon 

 in the Yale Collection, and a few more in the American and National Museums. None 

 of these is small enough to pertain the Ciniolomys, sens, lat., they are just the length 



^*This last right lower molar was considered as the last left upper molar by Marsh. Working 

 before the group was adequately known, Marsh was inevitably led into a number of similar misidentifi- 

 cations. It has not seemed necessary to correct them all formally here ; all are corrected by implication. 



