TRANSACTIONS OF WAGNER 



UINTA SELENODONTS 



well developed and forms a stout, incurved hook, roughened at the tip ; this 

 process is less prominent and massive than in Poefyrotheriuin, just as in the 

 latter it is smaller than in the existing camels and llamas. 



The humerus is, unfortunately, not preserved entire in any of the speci- 

 mens, the most complete one lacking the proximal end. It may be seen, how- 

 ever, that the bone is long, slender, and laterally compressed, very much 

 resembling the humerus of Poebrbtherium, though decidedly narrower when 

 viewed from the front; seen from the side it has the same sigmoid curvature 

 as in the latter genus. The proximal portion of the shaft has a considerable 

 fore-and-aft diameter, which gradually diminishes downward, while the distal 

 portion has but a moderate transverse expansion. The deltoid ridge is very 

 low and inconspicuous, even less prominent than in the White River genus, 

 and is in veiy marked contrast to the great ridge and hook of the modern 

 tylopodans. The supinator ridge is likewise feebly developed and adds little 

 to the breadth of the shaft. The anconeal fossa is small, narrow, and deep, 

 perforating the shaft and forming a minute supratrochlear foramen. The 

 trochlea is very narrow and set quite obliquely to the long axis of the shaft, 

 nearly as much so as in Poebrotherium. Considering the geological date of 

 Protylppus, the humeral trochlea shows quite an advanced type of structure ; 

 it has a considerable vertical diameter and is hour-glass shaped, grooved in 

 the middle for the corresponding ridge on the head of the radius; the inter- 

 condylar ridge, which is placed on the outer half of the trochlea, is very weak, 

 but quite distinct nevertheless. 



The bones of the fore-arm are in ordinary adults separate throughout, 

 although the radius has already begun to increase and the ulna to diminish, 

 and, as Wortman has shown, coossification takes place in aged individuals at 

 the middle of the shaft, leaving the proximal ends free. Of the radius I have 

 no well-preserved example. 



The ulna has undergone considerable reduction, and its articulation with 

 the humerus is altogether posterior. The olecranon is long, narrow trans- 

 versely, but thick antero-posteriorly, with the free end somewhat thickened 

 and club-shaped. The coronoid process is prominent, but the sigmoid notch 

 is rather shallow, for its distal part is incomplete. The shaft is long and quite 

 slender, tapering rapidly towards the distal end, and has a decided anterior 

 curvature. For most of its length it is trihedral, with sharp postero-external 

 and antero internal edges, but the distal portion loses the trihedral form and 

 becomes laterally compressed and very slender. 



