512 THE MAMMALIA OF THE UINTA FOKMATION. 



is found to possess four toes in the manus, tlie series can be readily placed interme- 

 diate between ILjracodon and the rhinoceroses ; (3) these j^enera have in common 

 a complete incisor series, but in contrast with Amyaodon, IJyracodon shows a decided 

 degeneration of the canines ; (4) the molars of Ilijracodon, while imitating those of 

 the rhinoceros, entirely lack the pecnliar proportions and inclinations of the extei-nal 

 and transverse crests, which are so distinctly rhinocerotic in Amyaodon. 



The difficulties which have been indicated in any attempt to derive the rhinoceros 

 from any known species of Amyaodon do not preclude this derivation from some 

 member of the Amyaodoatidcii ; the point insisted upon here is, that such a member 

 has not yet been discovered ; secondly, that the later Amynodontidfe represent a dis- 

 tinct line of pseudo-rhinoceroses, probably intermediate between the true line and the 

 Hyracodon series. They approach the true rhinoceros in part of their tooth structure 

 and probal^ly in the foot structure, and the latter series in the remainder of the den- 

 tal structure. 



DIPLACODON Marsh. 



As observed by Marsh at the time this genus was established,* Diplacodon is 

 intermediate between the Bridger genus Pcdoeosyops and the Lower Miocene Tltano- 

 tJieriidie. Nothing is known of the skull, the type consisting merely of the palate 

 and complete maxillary dental series. Unlike the Bridger genera the fourth premo- 

 lar is like the molars and the second and third premolars are developing the second 

 internal cusp, and in process of assuming the molar pattern. We may anticipate 

 that the skull will show the initial development of the great horns of Titaaotherlum. 

 The numerous remains of the skeleton in the Princeton collection enable us to fully 

 confirm the relationship suggested by Marsh. Tlie skeleton is in every detail inter- 

 mediate between that of Pala'osyops and Titaaothermm, and presents a remarkable 

 intermingling of characters persisting from its smaller Bridger progenitors and antici- 

 pating its great Miocene successor. Allowing for a moderate elongation of the dor- 

 sal spines, IHplacodmi stood about five feet seven inches at the shoulder, while Pakii- 

 osyops stood a little less than four feet, and Titaaotherlaia considerably ovci" seven 

 feet. The metapodials i-etained the stilted spreading character seen in Palceo^yops ; 

 much longer than in this genus and less bulky than in Titaaoiherinni. On the other 

 hand the cervical vertcbrte in some species arc much flattened, indicating a short neck 

 similar to that in the Miocene genus. The detailed comparative measurements given 

 below are taken from a Pala'osyops species of the middle size, probably P. pala- 

 dosus ; the measurements of Diplacodon are mostly from a skeleton belonging to a 



»Aiu. .Iiuir. Sci. aiul Ails, March, 1875, \\ S-IG. 



