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BULLETIN 171, UNITED STATES NATIONAL MUSEUM 



longitudinal ribbing so characteristic of these teeth in M. nasutus; (2) the form 

 of the third upper premolar (the only tooth which can at present be directly com- 

 pared) which is proportionately broader, while the secondary cuspules are much 

 less prominent; and (3) the relative distance between the cheek-tooth rows of the 

 opposite sides, which is much greater than in the type of Leidy's species. 



A second specimen (Cat. 8160 U.S.N. M. Coll.), consisting of three milk molars 

 and the first true molar of the right side [fig. 48], seems to belong to this species, 

 although the first molar, which is entirely unworn, is much narrower than the 

 corresponding tooth in the fully adult type specimen. This difference, however. 



FiauEE A7.—MylohyM exortitnis Qidley, type specimen (U.S.N.M. no. 8876): a, Superior cheek teeth; 

 b, inferior cheek teeth. Two-thirds natural size. Cumberland Cave Pleistocene, Maryland. 



may be due to the fact that in the young specimen this tooth had not reached the 

 stage where the roots had begun to form and is therefore so immature as not to 

 have yet acquired its maximum width. 



This specimen is especially interesting, since it affords a comparison of the milk 

 dentition with that of the type of M. pennsylvanicus. The unworn condition of 

 the deciduous teeth and the incomplete development of the first permanent molar 

 in this species indicate a somewhat younger individual than Leidy's type. It 

 differs from the latter in (1) the smaller size of the corresponding teeth (see table 

 of measurements), (2) in the narrower proportions of the milk-molars, and (3) 



Figure iS.—Mylohyus exortivus Qldley: Superior deciduous premolars and first permanent molar of right 

 side (U.S.N.M. no. 8160), occlusal view. Natural size. Cumberland Cave Pleistocene, Maryland. 



in the greater complication of secondary cuspules, especially in the anterior half 

 of each of these teeth. In this last feature they show a decided advance in com- 

 plexity of the milk-teeth over those of either of the living genera of peccaries. This 

 is less pronounced in M. pennsylvanicus but this species also has more complex 

 milk-molars than either Tayassu or Pecari. 



A comparison of the lower jaw of the type of M. exortivus with 

 illustrations of the Franks town Cave lower jaw referred by Peterson 

 (1926, p. 256, pi. 18) to Mylohyus pennsylvanicus shows the two to be 

 very nearly the same size, but a noticeable difference is seen in the 

 character of P2. In the Frankstown Cave form P2 is a simple tooth 

 with two cusps longitudinally arranged. In M. exortivus the tooth is 

 completely molariform though smaller and relatively narrower than 

 the succeeding teeth. 



