150 PATAGONIAN EXPEDITIONS: PALAEONTOLOGY. 
of the medium-sized ungulates generally. The anterior ribs have larger 
heads, larger and more prominent tubercles, straighter, broader and more 
compressed shafts. In the middle region of the thorax the ribs are longer 
and thicker and have a stronger outward curvature, while the tubercles are 
smaller and much less prominent. Posteriorly, the ribs become much 
more slender and rod-like and all of the ribs are conspicuously more 
slender than those of Toxodon. As a whole, the thorax is long, deep and 
capacious. 
The sternum (PI. XXII, figs. 3, 4) is not completely preserved in any 
of the specimens, but what remains is very peculiar. The presternum, or 
manubrium, is long, narrow and laterally compressed ; the facets for the 
first pair of ribs form prominent projections and in front of these is a long, 
much compressed and keel-like process, somewhat as in the horses and 
tapirs, but more pointed and having a downward direction. Behind the 
rib-facets, the presternum is much broader on the dorsal face, narrowing 
ventrally to an edge or keel. Four of the mesosternal segments are pre- 
served, of which the first three are depressed and flattened, though very 
broad, the width and dorso-ventral thickness being nearly equal. The 
fourth segment is much narrower and more compressed, contracting 
toward the hinder end, and in this segment the thickness greatly ex- 
ceeds the breadth. 
In Toxodon the sternum is of similar type, but with considerable differ- 
ences of form ; the manubrium is relatively longer and the rib-facets are 
not prominences, but deep, irregular concavities. All of the presternum 
is more depressed and flattened than in Nesodon and the anterior process 
is broad and low, ending in a blunt point, and quite different from the 
sharply compressed and keel-like process of the Santa Cruz genus. The 
mesosternal segments are broader and more massive than in the latter 
and the fourth segment is not contracted. 
Appendicular Skeleton . — The scapula (PI. XXII, figs. 1, 2) is very 
large, of high, narrow and subquadrate shape ; the spine pursues a some- 
what oblique course, from above downward and backward, making the 
postscapular fossa much wider than the prescapular proximally, while dis- 
tally these proportions are reversed. The glenoid cavity is large and 
broadly oval in form and the neck is very wide, made so by the cora- 
coid, which, when seen from the outer side, appears to be merely a portion 
of the scapula, but in distal or internal view is shown to be a massive in- 
