270 
PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 
don , there is no material difference of structure ; in Homalodontotherium 
the great development of the deltoid crest, the epicondyles and the supi- 
nator ridge is what gives the characteristic form. 
The fore-arm bones are separate and show no tendency to coossify. 
Of the radius, only the distal end is known, but, from the form of the 
capitellum of the humeral trochlea and the proximal end of the ulna, it 
may be inferred that the head of the radius was discoidal in shape and 
allowed considerable freedom of rotation. Further, the length of the ulna 
makes it very obvious that the radius must likewise have been very 
elongate. The distal end is extremely heavy and of broad, transversely 
oval shape ; on the outer side is a large, nearly plane facet for articula- 
tion with the ulna. The scaphoid facet is very broad transversely, narrow 
antero-posteriorly, and behind its internal moiety is a very large and deep 
pit completely encircled by bone. The lunar facet, which is very obscurely 
demarcated from that for the scaphoid, is as broad transversely as the 
latter and very much more so antero-posteriorly, covering the entire 
thickness of the distal end ; it is decidedly concave in both directions. 
The missing shaft may be reconstructed from these indications ; the 
proximal portion was evidently slender and crossed over the front of the 
ulna ; the shaft became heavier downward and was actually massive at 
the lower end. 
Of the ulna, I have examined a distal epiphysis in the Ameghino col- 
lection and an entire isolated bone in the Princeton Museum (No. 15,747), 
the lower end of which agrees closely, save in one unimportant particular, 
with the Ameghino specimen. I am inclined to believe that there is 
some mistake in the identification of the very fragmentary ulna which 
is figured by Lydekker (’93, PI. XX, fig. 4). The complete ulna shows 
that the fore-arm was extremely elongate and far longer than the leg, the 
disproportion being much as in Macrotherium , as figured by Filhol (sub 
Chalicotherium , ’91, PI. XLIII) but this is true only in very much less 
marked degree of the North American genus Movopus. In its whole 
character the ulna of Homalodontotherium is closely similar to that of 
Nesodon, but on a much larger scale and relatively far more elongate. 
The olecranon is long and is continued upward nearly in the line of the 
shaft, projecting backward even less than in Nesodon; the posterior 
border of the process is broadened and flattened in a way not seen in 
the latter genus, but the proximal end is only moderately thickened and 
