90 



BULLETIN OF THE 



which probably belongs to the A ceratherium of the lower beds. It differs 

 widely in its proportions from other specimens found in this collection, w r hich 

 belong either to the Aceratherium of the higher beds, or to Metamynodon. The 



best preserved specimen of this second type, 

 (marked a 3 ) is comparatively short and broad, 

 with spreading digits and rugose surfaces for 

 muscular attachment (Figure 13). The pro- 

 portions of the metapodials to the tarsals are 

 similar to those in Ceratorhinus. The calcaneum 

 has a powerful tuber ; the ectal astragalar facet is 

 very convex ; the sustentaculum is narrow, and 

 its oval facet is continuous with the inferior; 

 the cuboidal facet is nearly horizontal. About 

 one fifth of the astragalus rests upon the cuboid. 

 The relations of the cuboid, navicular, and ecto- 

 cuneiform repeat those observed in Rhinocerus. 

 The mesocuneiform is very short, giving mts. II. 

 a wide articulation with the ectocuneiform. The 

 metatarsals are powerful, the lateral pair having 

 approximately the same length as in E. indicus. 

 This type of foot is related directly to that of 

 Aphehps. 



The manus and pes of a third specimen (marked 

 a 6 ) show several interesting differences. In the 

 pes, the metatarsals are of the same proportions, 

 but the calcaneo-cuboidal facet is oblique and 

 narrow, resembling that in Hyracodon, and the sustentaculum is very small. 

 The remains of the carpus show that the species to which this specimen belonged 

 had a greatly reduced fifth digit, constituting a functionally tridactyl manus. 

 The evidence for this is in the greatly reduced lunar-magnum facet, which is 

 invariably characteristic of tridactylism. 1 



It may be noted here that among the carpals of Titanotherium there is a well 

 preserved lunar, which has its magnum facet much reduced anteriorly, so there 

 is little question that we shall yet discover a tridactyle species of the genus. 



Figure 13 — R ; ^ht pes of Ace 

 ratheriuni X ±. 



The Rhinoceros Molars. 



The peculiarities of the molars of Aphelops will be made more clear by a few 

 observations upon the molars of the rhinoceroses in general. The three main 

 crests of the lophodont crown may now be distinguished in part by terms which 

 express their homologies with the elements of the sextubercular superior and 

 quadritubercular inferior molars of the primitive ungulate, Phenacodus. In the 

 upper molars, the outer crest is formed by the union of the primitive paracone 



1 See Osborn, Mammalia of the Uinta Formation, p. 567. It is possible that these 

 feet belong to Metamynodon. 



