1 68 PATAGONIAN EXPEDITIONS: PALAEONTOLOGY. 
entirely suppressed ; (4) the shaft loses its rounded form and becomes 
much compressed antero-posteriorly and very flat on the posterior face ; 
(5) the medullary cavity disappears and its place is taken by cancellous 
bone ; (6) the condyles become less prominent and more nearly alike in 
size and shape. 
The patella of Nesodon (PI. XXIII, figs. 6, 7) is a curious and char- 
acteristic bone and quite different from that of Toxodon. It is very short 
proximo-distally, quite broad and very thick, especially on the proximal 
and internal sides, thinning much to the external border. The distal 
border is concave and its internal portion is slightly produced downward. 
The anterior surface is very irregular and rugose and the posterior surface 
is obscurely divided by a low ridge into a narrow external, and much 
broader internal facet for the rotular groove of the femur. In Toxodon , 
on the other hand, the patella is short, of no great thickness, but im- 
mensely broad ; the internal portion is greatly extended and recurved, so 
as to cover the inner face of the rotular prominence, while the distal 
border is deeply notched in a very characteristic fashion. 
In Nesodon the leg-bones (PI. XXV, figs. 1-3) are ankylosed at the 
proximal, but not at the distal end, a very peculiar arrangement. Except 
at the ends, the two bones are separated by a very wide interosseous 
space. The tibia is a little shorter than the femur and has a broad thick 
head, which projects far external to the shaft. The outer condyle is 
oblique and nearly plane transversely, very slightly concave antero- 
posteriorly, while the inner condyle is more concave and its portion of 
the bifid spine is decidedly more prominent. The shaft is surprisingly 
slender and its proximal portion is so strongly compressed laterally as to 
lose the trihedral shape common to almost all terrestrial mammals. The 
cnemial crest is prominent and heavy, but short, and its proximal end is 
deeply impressed for the insertion of the patellar ligament. On the 
external face of the shaft is a long interosseous crest, which, however, is 
prominent and rugose only for a short distance about the middle of its 
course. The distal half of the shaft is less compressed and more rounded 
than the proximal moiety and the distal end is moderately expanded and 
heavy. The surface for the astragalus is nearly square and is obscurely 
divided into a broader and shallower external, and a narrower, deeper 
internal facet. The intercondylar ridge is very low, but forms a tongue 
on each border, that on the dorsal side being the narrower and more 
