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PATAGONIAN EXPEDITIONS : PALEONTOLOGY. 
more strongly convex than in Nesodon. In brief, the humerus of the 
Pampean genus is a far more massive bone and its processes for mus- 
cular and ligamentous attachment are better developed, heavier and more 
prominent than in the Santa Cruz animal, a change which corresponds 
to the general increase in size and bulk of the former. 
The fore-arm bones are always separate, not displaying the slightest 
tendency to coossification, and are crossed, the radius passing from the 
external to the internal side of the arm ; the two bones are in contact, not 
only at the proximal and distal ends, but also for some distance along 
the middle portion of the shafts, by means of interosseous crests. Thus, 
the radio-cubital arcade is divided into a shorter proximal portion, which 
is very narrow and slit-like, and a longer, wider distal portion. 
Relatively, the radius (PI. XXV, figs. 5-8) is elongate and quite slender ; 
the head is very small, and transversely oval in form and covers but a 
small part of the humeral trochlea ; the shape of the articular surface varies 
somewhat in different individuals, though possibly these differences should 
be regarded as specific rather than individual. In some specimens the 
surface is simply concave ; in others it is divided into a larger external 
concavity and a smaller, internal, saddle-shaped portion. The proximal 
facet for the ulna is a narrow band on the inner side of the head. On 
the external side of the head is an articular surface, to which is attached 
a large sesamoid bone, shaped like a small patella and apparently devel- 
oped in one of the extensor tendons of the digits. No bicipital tubercle 
is visible on the radius. The shaft is slender, irregular and with a forward 
curvature, increasing in size toward the distal end ; the interosseous crest 
is distinct and very rough, though of no great prominence, and runs for 
nearly the whole length of the shaft. The distal end is quite massive, both 
broad and thick ; the styloid process is long and prominent and con- 
tinues distally the facet for the scaphoid, which conspicuously notches the 
dorsal border of the distal end of the radius. The lunar surface is con- 
cave and is considerably deeper palmo-dorsally than that for the scaphoid. 
The distal facet for the ulna is very small and oblique, presenting proxi- 
mally almost as much as laterally. 
The ulna (PI. XXV, figs. 4, 5) is much heavier than the radius, except 
near the distal end. The olecranon, though long, projects but little 
behind the plane of the shaft, from which it slopes up very gradually, 
and the free end is only moderately thickened and rugose. The facets 
