536 THE MAMMALIA. OF THE UINTA FORMATION. 



the scaphoid aud hinar was probably concomitant." In the earliest known Diplar- 

 thra the scaphoid has a facet upon the magnum of the same width as that of the un- 

 ciform beneath the hmar — these facets increase, pari passu, until we reach the most 

 specialized forms. In all the Perissodactyla the law of ectal displacement of the 

 metacarpals is maintained ; after mc. i disappears the trapezium persists ; as mc. ii is 

 reduced it invariably retains its entire hold upon the trapezoid ; mc. iii sj^reading 

 only to the ectal side (Kowalevsky). In the ada])tively reduced Artiodactyla, how- 

 ever, mc. Ill acquires a broad trapezoidal facet, forming an exception to the otherwise 

 universal rule. 



The general modifications of the Pes are still more interesting. In the astragalus 

 the flexor foramen disappeared, being for a long time represented by a groove ; 

 the primitive foot undoubtedly had a limited grasping power, the flexion being dis- 

 tributed between the metapodials and phalanges ; this was replaced by the greatly in- 

 creased play in the ankle joint and the development of the muscles inseited in the tendo 

 acliillis as propellers. The ti'ochlear groove deepened, limiting the motion to the fore 

 and aft direction (Cope) ; the navicular facet became saddle-shaped and then flat, ex- 

 cepting in the artiodaetyl phylum ; the sustentacular facet divided into two, which may 

 be called the "sustentacular" and "inferior ," producing three distinct cal- 

 caneal facets.f The displacement of the astragalus and cuboid by reciprocal growth 

 was almost universal, having but two exceptions, in the Proboscidia, where the navic- 

 ular extended upon the cuboid, and in the lines of Perissodactyla, which tended to 

 monodactylism. The calcaneum in several series acquired a fibular facet, the tuber 

 was elongated and the sustentaculum extended inwards. While rarely in contact 

 with the navicular anterioi'ly (as in the primitive horses), the calcaneum in some 



"* Upon this i)oinl hangs the question whether the diplarthrous toot ever passed through the amblypodous stage, 

 as held liy Cope. In the absence of any direct palfeontological evidence, Schlosser (Beitriige z. Stammesgeschiehte der 

 Ilufthiere, p. 6) has strongly insisted upon the direct derivation from the Condylarthra upon the ground of the dis- 

 similarity both in foot and tooth structure. His objections, though well taken, indicate a misunderstanding of Prof. 

 Cope's ta.\onomic principle of selecting genera at a certain stage of evolution in foot or tooth structure, to form a family 

 or order, irrespective of actual descent or relationship. When, upon this system, the derivation in question is main- 

 tained, no genetic relationship of the Perissodactyla aud known Amblypoda is thereby implied, but that at one stage 

 in the evolution of the diplarthrous foot the lunar rested widely upon the unciform, as in Coryphodon, the scaphoid 

 not reaching the magnum. Tliis jiroposition .seems to us highly improbable, for we must suppose tliat the lunar first 

 extended upon the unciform while retaining its entire magnum facet, then it became reduced while the scajihoid ex- 

 tended upon the maguuiu— this hypothesis is rendered improbable by the fact that in all the early Diplarthra the 

 scapho-magiiuiM and lunar uncitbrm facets are subeciual. The proboscidian manus is an e.xact counterpart to the 

 amblypod manus, and shows that tlu' latter is to be considered not as in nii intermediate Imt as in a linal stage of de- 

 velopment. 



I It is intciTstiiig to note the close paralh'lisiii to tlic-^e adaptations to digit igiadism in the Carnivora ; we oliserve 

 an entirely analogous division in these facets and evolution of tlie anUlc Joint. 



