146 PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 
behind the transverse processes, so that the posterior face is subcircular 
and slightly concave ; the ventral keel is very obscurely marked and the 
anterior cotyles for the atlas are very wide and low and feebly convex. 
The odontoid process is rather long, very stout and somewhat compressed 
laterally, giving a vertically oval section. The pedicles of the neural arch 
are narrow antero-posteriorly, but thick and heavy, and the neural canal 
is rather small. The neural spine is hatchet-shaped and not very large, 
projecting forward but little in advance of the pedicles of the arch ; the free 
border of the spine is thick and rugose, especially behind, where it forms 
quite a massive tuber. The postzygapophyses are large and prominent, 
presenting almost laterally, and are somewhat convex dorso-ventrally. 
The transverse processes are short, slender and thin, but diverge strongly 
from the sides of the centrum ; they are perforated by the very large 
canal for the vertebral artery. 
In Toxocion the axis differs from that of Nesodon in only a few details, 
except, of course, its very much greater size and massiveness. The an- 
terior cotyles are more saddle-shaped ; the odontoid process is relatively 
shorter and heavier; the neural canal is higher dorso-ventrally and the 
spine more massive ; the postzygapophyses are of a different shape, with 
the long diameter directed antero-posteriorly instead of dorso-ventrally, 
and presenting more obliquely downward ; the vertebrarterial canal is de- 
cidedly smaller. 
The third cervical has a short, heavy centrum and long, slender, de- 
pressed transverse processes, which are quite simple, showing no indica- 
tion of the inferior lamella, and are perforated at the base by the very 
large canal for the vertebral artery. The neural canal is quite small and 
the arch is broad and nearly flat on the dorsal surface. In all of the 
available specimens the neural spine is broken and its full height cannot 
be determined, but it evidently was unusually prominent. The prezyga- 
pophyses are large and present internally, with almost vertical faces, 
while the postzygapophyses present obliquely downward. 
The succeeding cervical vertebrae have short, heavy and slightly opis- 
thocoelous centra and prominent transverse processes. The inferior 
lamella first appears on the fourth vertebra, is considerably larger on the 
fifth and on the sixth becomes a great, hatchet-shaped plate, which is 
produced far posteriorly (PI. XXIV, fig. 3). In all, including the sixth, 
the vertebrarterial canal is very large and conspicuous ; the transverse 
