306 PROCEEDINGS OF THE ACADEMY OF [1892. 



MESONYXi Cope. 

 Syn. Sy)ioplothermt)i- Cope, DroniocyoTv" Mar>h. 



I have elsewhere^ given a very full acount of the osteology of 

 this genus, and shall therefore devote but a small space to it here> 

 emphasizing especially the points in which it differs from Pachymna' 

 The dental formula is If Ct Pf Mf. The u])per molars differ from 

 those of Pachycena in the equal size of the para- and metacones, 

 and P^ is even more completely molarifbrm, all three cusps being of 

 nearly equal size. In the lower jaw the premolars, except p^, have 

 assumed the niolar pattern by the addition of the paraconid and 

 trenchant talon. The molars differ from those of Pachycena in the 

 reduction of the paraconid and enlargement of the talon, and in 

 the simplification of m^, in which the paraconid is rudimentary or 

 absent and the talon very small. The humerus lacks the entepi- 

 condylar foramen and has a remarkably well developed trochlea, 

 which is high, thick and with very large intertrochlear 2:)rominence, 

 so as to resemble the corresponding part of a perissodactyl. The 

 ilium is like that of the bears and much more expanded than in any 

 other known creodont. The hind limb does not much exceed the fore 

 limb in length. The feet are digitigrade and tetradactyl, the meta- 

 podials symmetrically arranged as in the hytena and the metacar- 

 pals very completely interlocked. In this genus the size of the 

 head and length of trunk are very great, as compared with the 

 length of the limbs and feet. 



Three species of the genus have been described : M. obtusklens 

 Cope (Dromocyon vorax Marsh), M. (Syiioplotherium) lanius Cope, 

 which are from the Bridger, and M. ulnteiisis Scott, from the Uinta, 

 a much larger species. To these may be provisionally added a 

 fourth which was obtained by the Princeton expedition of 1890 in 

 the White River bad lands of South Dakota. The specimen con- 

 sists of a fore leg, unfortunately not associated with teeth and there- 

 fore its reference to this genus cannot be considered final, though 

 there can be little doubt that it belongs to a member of the family. 

 The species may be named and described as follows: 



TMesonyx dakotensis, t^p. nov. 



Size greater than that of either of the Bridger species, less than 31. 

 uintensis. The distal end of the humerus is broader and more flat- 



1 Proc. Am. Phil. Soc, 1872, p, 460. 



2 Loc. cit. p. 483 ; referred to Mesonyx, Tert. Vert. p. 358. 



3 Am. Journ. Sci. and Arts 3d Ser., Vol. XII, 1876, p. 403. 

 * Journ. Acad. Nat. Sci. Phila., Vol. IX, no. 2. 



