No. i.] OSTEOLOGY OF PCEBROTHERIUM. 33 



The humeral facets of the ulna are peculiar in the Tylopoda, 

 and quite different from those which occur in the ordinary rumi- 

 nants : the peculiarity is already well established in PcebrotJie- 

 rium. Externally the articular surface is confined to the superior 

 border of the sigmoid notch, whereas on the inner border it is 

 continued down the anterior face of the notch, and passes with- 

 out interruption into the corresponding facet of the radius. 

 The olecranon is high, but compressed and not massive ; it is 

 continued upward in almost the same straight line as the shaft, 

 and is abruptly truncated at the end, its superior margin sloping 

 downward and forward, and deeply grooved by a sulcus for the 

 extensor tendons. Both the anterior and posterior margins of 

 the olecranon are nearly vertical. In Camelus and Auclienia the 

 olecranon is very different, in that it projects strongly behind 

 the line of the shaft ; and the superior margin is rounded, 

 rugose, and not grooved by a tendinal sulcus. 



The shaft of the ulna is much reduced, but decidedly less so 

 than in the modern forms, in which it is almost entirely poste- 

 rior in position, and even the distal end is concealed by the 

 expansion of the radius. In Pcebrotlierium, on the other hand, 

 the shaft is visible from the front for its whole length, and when 

 viewed from behind covers half the width of the radial shaft. 

 The distal portion of the ulna is enlarged both in thickness and 

 depth, fitting into a deep depression of the radius just above its 

 distal expansion. From this point the ulna tapers rapidly to 

 its distal end, terminating in a small tip which projects some- 

 what below the level of the radius, and showing facets for the 

 cuneiform and pisiform, which in old animals pass into the cor- 

 responding facets of the radius without any visible line of 

 separation. 



The carpus (PI. II, Figs. 33-35) has been figured, and briefly 

 described by Cope, who has called attention to its more impor- 

 tant characters, but a more detailed account is necessary for 

 the purpose which we have in view. As a whole the carpus is 

 broad and low — a change which has chiefly affected the distal 

 row, as the height of the proximal series is still considerable. 

 The scaphoid is narrow but deep antero-posteriorly ; its radial 

 surface forms a low broad ridge in front, and behind a deep con- 

 cavity, terminating in an elevated hook, which extends well up 

 upon the posterior surface of the radius, and necessitates the 



