TIIK M.V.MMALIA OF THK UINTA FORMATION. Gil 



tion, viz., that this is anothiM' line of pseudo-rhinoceroses parallel to the JIi/iaco(h)ii- 

 tiike. For while the molar dentition and [icrliaps also tlir lower canines point 

 directly to Acevatherium, the development of the other teeth and man\- cliaracters of 

 the skull are very diverse. Prof. Cope* i)resents a third alternative in deriving 

 I[ijrac(Ml(>ii from Amynodon, placing these genera in one family, and not placing 

 the latter in the Aceratlierinni {^Cwiiopu-i) series at all. 



The features which unite the known AinynodoididtH with the aceratherine line 

 are: (I) The structure of the lower molars and of the superior molars to the 

 second molar inclusive ; (2) the structure of the astragalus, in extending upon the 

 cuboid. The dietiugnishing features of the Amynodontidx are: (1) The retention 

 of fully functional incisors in both jaws, and the great enlargement of the canines; 

 (2) the retention of the lophiodont character of the last ui)per molar in its sulxpiad- 

 i"ate form and complete cvternal crest ; (3) the retarded development of the postei'ior 

 crests of the superior premolars ; (4) the deep excavation of the maxillaries in front 

 of the orbit and flattenin"r of the cranium, tofjether with the shorteninof of the nasals. 



In Aceratherium, on the other hand, from the lowest American Miocene, we fuid 

 the upper canines entirely wanting, also one of the incisors; the premaxillaries are 

 narrow and weak, and support two subequal incisors. In the lowTr jaw^ are the large 

 lateral teeth and small median pair, of very doubtful homology. Upon the discovery 

 of ^Imynodon, Prof Marsh suggested that the outermost lower tooth of ^li'cra- 

 therium was a canine, instead of an incisor as previously supposed, but there is little 

 additional basis for this opinion. A. occidentale has, moreover, long nasals and con- 

 vex maxillaries. In fact, excepting in the molar series we lind no support for the 

 supposition that Aceratherinin is a descendant of any known species of Aiinjnodon. 

 The resemblance of Hyracodon to llyrarhyinA, of Mcxohiyims to Jfymrolherhim, of 

 Titiinotherlian to I'ali'()!<ynps is altogether of a different character, and in each ca.se 

 is relatively far more signilicant of direct descent. 



So far as Prof Cope's suggestion of the descent of Ifyracodmi from Aunjiioduii, 

 is concerned, the following ol))ections may be made: (1) The dentition and o.stcology 

 of llyrnchyii.-i fill in every respect the conditions which we should expect to find in 

 an ancestral foi-m of /////v/ro/A^//, boili in the displacement and reduction of the foot 

 bones and characters of the teeth ; (2) a wide dillerence between the aceratherinc 

 and hyracodon series is found in the structure of the carpus and tarsus. The posi- 

 tion of the Atnyiiodiiididin it appears turns ui)on these parts. An astragalus found 

 near the skull of .1. rndif/itn.i (see Diag. 12) resembles closely that of Acerathnnun 

 in its broad cuboidal facet. If, in addition, as we think highly prol)ablc, Ainynodon 



•Tlif IVrissodnctyli. Am. Niil., 1S87, \<. 9!)U. 



