AKSrNOlTIIERIUM. 
37 
rigid. Immediately beneath the antero-external angle of the upper end there is a 
prominent bicipital tuberosity {h.t.). Beneatli this the sliaft contracts rapidly, 
and its slenderest point is about one-third of its total length from the proximal 
articulation. Here the anterior face is slightly concave from side to side, while 
the posterior is convex with a median ridge (rJ). Distally a great expansion of the 
bone occurs. The sharp antero-external border of the shaft is continued down 
the outer face (r.) (that is, outer as regards the ulna) of the expansion, and terminates 
in a large roughened prominence (p-p.) which is situated at the junction of the 
shaft and epiphysis and is separated from the posterior portion of the articulation 
by a well-marked depression. The inner edge of the shaft is continued down 
as the posterior border of the distal expansion and becomes widened out into a 
roughened surface. Lastly, the ridge {rJ) above mentioned as occurring in the 
middle of the posterior surface, runs forwards and is prolonged into a great flange-like 
expansion {a.p.), the anterior edge of which forms the front border of this part of 
the hone, and is continued down to the distal articulation. The ulnar side of the 
expanded portion of the bone is concave and was closely applied to the inner face 
of the distal end of the ulna, for union with which there is also a narrow facet 
along the lower edge of the bone (see figure, the lower of the two lines from u.s.'). 
The distal articulation, though much smaller than that of the ulna, is of 
considerable antero-posterior extent ; posteriorly it consists of a large rounded 
head (1.) which passes anteriorly into a concave surface looking downwards and 
somewhat outwards. The inner (preaxial) portion of the head is marked off from 
the rest by a very faint ridge, and is the comparatively small surface for articulation 
with the scaphoid. The remainder articulates with the inner portion of the lunar (1.). 
The general form of this distal articulation is very similar to that of the radius of 
Elephas, which differs only in the following points: — (1) the posterior portion of 
the articulation is more cylindrical and less ball-like in form, owing to the lesser 
degree of convexity in a transverse direction ; (2) the anterior part of the 
articulation is smaller, less concave, and less clearly marked off from the posterior 
part. The line of division between the scaphoid and lunar surfaces is the same 
in both cases. In Elephas the radial articulation with the carpus is much larger 
compared with the ulnar portion than in Arsinoitherium. 
The distal end of the radius of Uintatherium is of very different form : it consists 
of two concave surfaces, one for the scaphoid and the other for the pre-axial part of 
the lunar, separated by a sharp ridge. The distal end of the radius of BarytlLerium 
is almost identical with that of Uintatherium. 
In the ulna (text-figs. 18, 19) the olecranon process {ol.) is very large and greatly 
thickened posteriorly, so that its upper end forms a roughened mass of bone 
which projects somewhat to the outer, but still more to the inner side, which is 
also the higher. The articulation for the humerus is triradiate ; the upper (olecranon) 
