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35 

 UINTA SELENODONTS 



The manus (Plate II., fig. 9) is highly characteristic and quite different 

 from that of Poebrotherium. 



Of the carpus only the pyramidal, pisiform, magnum, and unciform are 

 preserved, but these suffice to show some of the more important features of 

 carpal structure. In the existing Tylopoda the carpus as a whole is broad 

 and low, which is due to the shortening of the distal elements, the proximal 

 row retaining considerable proximo-distal height. Already in Poebrotherium 

 this modification makes itself apparent, but in Protylopus it has hardly begun, 

 the magnum and especially the unciform being quite high in proportion to 

 their width. The pyramidal is high, narrow, and thick, and has much the 

 same shape as in Poebrotherium ; it is highest on the radial side, descending 

 steeply towards the ulnar side. In the last-named genus this carpal is quite 

 extensively covered by the radius, and in the existing Tylopoda nearly the 

 whole proximal surface is so occupied. So far as I can make out, this 

 change has already begun in Protylopus, but if so, the contact with the radius 

 is much smaller than in the White River genus. While the ulnar facet is of 

 the usual saddle-shape, that for the pisiform is small and confined to the 

 postero-external angle of the bone. 



The pisiform is shaped much as in Poebrotherium, but is longer, more 

 slender, and less thickened at the free end. Its proximal facets for the ulna 

 and pyramidal are not well preserved in the only available specimen, but they 

 meet at a more acute angle and make the proximal end more pointed than in 

 the White River form. 



The magnum is a small bone, low, broad, and thick, but its breadth 

 does not exceed its height so much as in the later Tylopoda, and, compared 

 with the other carpals, it must be regarded as rather narrow ; the head is 

 small and does not rise above the level of the dorsal portion. The scaphoid 

 facet is larger and more entirely proximal than that for the lunar, while in 

 Poebrotherium the lunar facet is relatively larger and less oblique than in the 

 Uinta genus. A comparison of the wedge-shaped space between the mag- 

 num and unciform in the two genera shows that in Protylopus the lunar must 

 have had a narrower distal beak, which descended almost to the head of the 

 third metacarpal. On the radial side the magnum bears a small facet for 

 the trapezoid, but I can discover none for the second metacarpal. 



The unciform is quite a large bone, exceeding the magnum in every 

 dimension ; it is relatively higher and narrower than in Poebrotherium, though 

 the difference is not very striking. Most of the proximal end is occupied by 



