2l8 
PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 
arrangement seen in Nesodon , in which the great length of the anterior 
thoracic spines produces a considerable hump at the shoulders. In Adi- 
notherium also the posterior thoracic and lumbar spines are broader and 
lower. The thorax is shallower and less capacious in the latter and the 
ribs more slender and less strongly arched outward. 
The scapula differs more from that of Nesodon in appearance than does 
any other bone of the skeleton ; the blade is narrower and more recurved 
and the spine also pursues a more curved course, running nearly parallel 
with the coracoid border. But the chief difference lies in the position 
and shape of the two very conspicuous metacromial processes, which in 
Adinotherium are closer together and the proximal one is smaller and 
more triangular, while the distal one is very much longer and narrower 
than in Nesodon. The pelvis also is longer and narrower than in the 
latter, the ilium having a longer neck and less expanded plate and the 
ischium being more elongate and slender. The limbs are relatively 
shorter, so that Adinotherium was proportionately a longer and lower 
animal, but the limb- and foot-bones are almost copies, on a smaller and 
lighter scale, of those of Nesodon , though there are several differences in the 
details of structure. For the most part, however, these minor differences 
are such as may properly be referred to the merely mechanical conditions 
of a rather massive and a small, light animal, and are hardly visible in a 
general view of the two skeletons, though obvious enough when the 
bones are separately compared. A not inconspicuous difference is in the 
character of the femur, which in Adinotherium is more slender, less flat- 
tened, and has a more prominent third trochanter ; the calcaneum also 
has a longer and less massive tuber, which lends a somewhat different 
appearance to the pes. 
This comparison has reference entirely to the two commonest species 
of their respective genera, A. ovinum and N. imbricatus. No doubt, 
other and greater differences might be enumerated, were the comparison 
extended to several species of each genus, but for this purpose the 
material now available is not sufficient. 
Species. — The lack of accurate knowledge of the stratigraphy of the 
Santa Cruz beds is an insuperable obstacle to any satisfactory discrimina- 
tion of the species of almost any genus of the Santa Cruz fauna and the 
toxodonts offer nearly as difficult a problem in this regard as do the 
gravigrade edentates. At present, we have but very inadequate means 
