528 THE jrAMMALIA OT THE UINTA FORMATION. 



extremely reduced. The trapezoid is wanting. The anterior face of the magnum is 

 subquadrate, since the ectal lunar and unciform facets are in the same slightly oblique 

 line. The unciform is remarkable for its depth; superiorly it has subequal 

 cuneiform and lunar facets ; inferiorly it has subequal mc. iii and iv facets and a 

 small facet for the rudiment of mc. v. The metacarpals have sharp distal keels 

 confined to the deep posterior faces. The phalanges are deep and elongate. 



The tarsus (Fig. 9). The main features of both the carpus and tarsus are de- 

 scribed in the section on the feet of the Perissodactyla. In addition the following fea- 

 tures may be mentioned : The sustentaculum of the c a 1 c a n e u m is very narrow ; 

 the cuboidal facet slants obliquely downwards as in the Artiodactyla. The metatar- 

 sals have the proportions seen in MesoMiyp^is, 



Measurements may be obtained from the natural size figures upon Plate XI, 



Tim Systematic Position of Triplojrus. 



Prof. Cope (Tert. Vert., p. 678; Am. Naturalist, 1887, p. 909) has placed Trij}- 

 lojms in a distinct family. He says : " The entirely rudimentary character of the 

 fifth metacarpal, which with its digit is so well developed in Hi/nicJryus, places 

 TrtplojJits in another family and in a distinct line of descent. I think that it must be 

 regarded as one of the forms connecting the lophiodonts with the rhinoceroses." We 

 cannot accept this conclusion. In the discussion of the manus and pes in the Hyra- 

 chyus-Uyracodon series, on a later page, it is shown that the feet of Triplop'm are in 

 every detail intermediate in structure between those of the above Bridger and White 

 River genera. The teeth are also transitional. In the upper molar scries of Triplo- 

 jynn, from the thiid premolar to the last molar inclusive, the external faces have a 

 striking resemblance to those of Ilyracodon, while the wearing surfaces of the same 

 teeth are close to those of Hyrachyits. The departure from the Hyrachyus pattern is 

 seen, first, in the decided elevation of the crests and disappearance of the postero- 

 external lobe or ridge ; second, in the rhinocerotic shortening of the posterior trans- 

 verse crest of m. '■ ; both these changes are in the direction of Ilyracodon. 



These characters suggest our considering the genus Triplopus as a transition 

 form. !None of the known species, however, exactly fill the requisite transition 

 features. T. nih'tah's and 7T oUiqnidens are apparently to be considei'ed aside from 

 the main line, on the following grounds : The fore ai-m is proportionally much longer, 

 the metapodials are more compressed, the lateral elements of the pes especially are 

 more reduced, than in any possible diiect ancestor of H. nehrascencis. A smaller, 

 lighter species of Ilyracodon is, however, represented in the Harvard collection by a 

 carpus which while slightly larger than that of T. ohliquidens presents a very similar 



