144 
PATAGONIAN EXPEDITIONS ! PALAEONTOLOGY. 
not so prominent, but there is a distinct postorbital angulation of the 
jugals. The zygomatic arches are of similar plate-like and laterally com- 
pressed character, but have a relatively greater dorso-ventral depth, 
especially the jugal portion ; posteriorly, the arches do not rise so high 
upon the lambdoid crest. 
The nasals are much shorter, giving an entirely different shape to the 
anterior nares, which are no longer terminal, but very oblique, presenting 
dorsally as much as forward. The two premaxillae are firmly coossified 
and the dorsal portion of their symphysis is raised into a prominent, mas- 
sive and rugose crest, which is much longer and more conspicuous than 
in Nesodon. The muzzle broadens anteriorly very much more strongly 
than in the latter, but the face, especially the edentulous part, is propor- 
tionately shallower dorso-ventrally. The hard palate is very much alike 
in the two genera, but in Toxodon it is narrower at the facial constriction 
and widens more posteriorly, and anteriorly much more. The posterior 
nares are placed farther back, as the palatines remain in contact in the 
median line for a greater distance. 
The mandible has a very different appearance in the two genera. In 
Toxodon the symphysis is much longer, extending back as far as and, 
though deeply concave, is shallower anteriorly and has no anterior wall, 
owing to the complete procumbency of the incisors. The jaw narrows 
forward to p4, and thence expands strongly, becoming very broad at the 
incisive alveolus, and there is no rise at the chin, the whole anterior 
region of the mandible being broad, depressed and with nearly flat ven- 
tral surface. The ascending ramus is very high and rather narrow and 
the curve of the angle rises more than in Nesodon , without the down- 
ward curvature seen in the latter. The sigmoid notch is almost obsolete 
and the coronoid low and weak, not rising to the level of the condyle. 
The latter projects internally more than in Nesodon , but has no such 
dependent, plate-like process from the inner end. 
Vertebral Column , Ribs and Sternum (PI. XII, fig. 2). — The vertebral 
formula is not definitely known, since no individual has yet been ob- 
tained with completely preserved back-bone. A careful examination of 
the available material leads me to the conclusion that the formula was 
nearly the same as in Toxodon , viz., C. 7, Th. 16-17, L. 4-5, though there 
is some reason to think that the lumbar region was relatively longer than 
in the Pampean genus, at least in certain individuals. 
