206 THE MERYCOIDODONTID^ 



Dentition: In addition to the size, which is much larger in comparison with M. profectus, P 4 

 and M 1 are proportionally much larger, while P 3 and M 2 are much nearer in size to the correspond- 

 ing teeth in the other species. The posterior crescent is much larger in P 3 and has almost completely 

 closed the posterior basin, while the anterointernal tubercle is larger and bounded lingually by a 

 complete cingulum. The external surface has a well-marked cingulum, and the anteroexternal 

 crest is far more prominent. All the above characters differ from those found in the P 3 of 

 M. profectus. P 4 differs from that in the Matthew and Cook species in having a very prominent 

 and complete internal cingulum, as well as in being proportionally greater in size. That tooth is set 

 markedly inward at its posteroexternal angle, whereas in M. profectus this same angle of P 4 is 

 almost exactly on a line with the outer edge of M 1 . M 1 differs in its proportionally greater size, and 

 in having a much better developed internal cingulum, a much larger parastyle, and a prominent 

 external cingulum. M 2 has a thinner parastyle, a thinner mesostyle, and well-developed internal 

 and external cingula, differing from those of the tooth in the other species. 



Discussion : Matthew ( 1 924A, p. 181) placed M. profectus as a synonym of M. major and 

 considered the two forms identical. To this I do not agree, since, as I have already pointed out, 

 there are very distinct differences between the two forms. I do agree with Matthew that there is no 

 question of the close alliance between these two species nor of the generic distinctness of Leidy's 

 species, as well as Matthew and Cook's, from the more or less contemporaneous genera, such as 

 Merycochosrus, Merychyus, Pronomotherium , and Ticholeptus. 



Leidy (1869, p. 121) wrote, in describing the holotype of this species: 



A singular peculiarity in the teeth of this specimen is the apparent absence of enamel in certain positions in 

 which it ordinarily exists before it is removed by attrition. Thus it appears to be absent on the external surfaces of 

 the inner lobes of the true molars. The enamel on the internal surfaces of the outer lobes thins away and appears 

 to cease at their lateral borders, or turns for a short distance over the contiguous edges of the external surfaces 

 of the inner lobes. The enamel also appears absent on the external surface of the inner lobe of the last premolar, 

 and for some distance at the sides of the internal surface of the outer lobe. In the third premolar the enamel 

 appears absent on the external surface of the postero-internal sub-lobe, and the greater extent of surface bounding 

 the interspace externally between the sub-lobe and the principal lobe. Where the enamel appears to be absent, 

 it is no doubt present in a much thinned condition, so as not to be obvious under ordinary inspection. 



Metoreodon? medius (Leidy) 1858 

 PI. XXXVII, figs. 14-16 



Original Reference: Notice of remains of extinct Vertebrata, from the valley of the Niobrara River. 

 Proc. Acad. Nat. Sci. Phila., X, p. 26 {Merychyus medius). 



Type Locality: Niobrara River, Nebraska. 



Geologic Horizon: Upper Miocene (Bed F of Hayden). 



Type: Holotype, Cat. No. 118 U.S.N.M., fragment of left lower jaw containing three molars, an isolated 

 M 3 , and an isolated superior canine. 



Specific Characters and Discussion: The type is so scanty and the three molars are so much 

 worn that it is difficult to know in what genus this form belongs. It is placed in Metoreodon, with 

 reservations. 



The teeth are more hypsodont than in Merycochcerus, Pronomotherium, or Mesoreodon, and 

 fully as much so as in Merychyus or Ticholeptus. The metastylid of M 3 is much smaller than in 

 Merycochcerus, M 2 and M 3 meet on a line at the inner edge, the metaconid of M 3 is not swollen 

 posteriorly, and the hypoconid of M 2 is much smaller in proportion to the protoconid than in that 

 genus. 



The type is much too large for Merychyus as now defined (about half again as large as 

 M. elegans), and Loomis says M. ? medius had stocky limbs and no facial vacuities. The molars are 

 wider in proportion to length, the protoconid is smaller than the hypoconid in Mi and M 2 , and the 



