148 
PATAGONIAN EXPEDITIONS : PALEONTOLOGY, 
for the rib-heads, which extend upward upon the base of the trans- 
verse processes ; the latter are very prominent and heavy and carry 
large, deeply concave facets for the rib-tubercles. The second thoracic 
vertebra has prezygapophyses of the cervical type, arising from the dor- 
sal side of the transverse processes, but the postzygapophyses occupy the 
normal position on the ventral face of the overhanging neural arch, the 
pedicles of which are deeply incised for the passage of the spinal nerves, 
while the third and succeeding thoracic vertebrae have the pedicles per- 
forated by foramina for the transmission of the nerves. The neural 
spines of the first four or five thoracics are exceedingly elongate and 
have a strong backward inclination. The rapid diminution posteriorly 
in the length of the spines produces a decided hump at the shoulders, 
though this feature is less strongly marked than in Toxodon , in which it 
is very striking. 
In the middle thoracic region the vertebrae have centra which in size 
and form differ but little from those of the anterior region, though the 
facets for the heads of the ribs become smaller and less concave and 
have a more inferior position. It is perhaps unnecessary to say that the 
change is a gradual one. The transverse processes are shorter, but still 
massive, and bear much smaller and nearly plane facets for the rib 
tubercles, and prominent metapophyses arise from the dorsal border. 
The neural canal is remarkably small and the pedicles of the arch are 
perforated by foramina for the nerves ; the spines become much shorter 
and more slender and have a less decided backward inclination than in 
the anterior region. 
In the posterior thoracic region (PI. XXIII, fig. 9) the vertebrae gradu- 
ally take on the lumbar type ; the neural spines are short, broad and 
plate-like, with thickened and rugose tips. The anticlinal vertebra 
appears to be the antepenultimate thoracic and on the last two the spines 
have a slight forward inclination. The small neural canal and the per- 
forated pedicles of the arch are retained throughout the region. Only on 
the last two vertebrae are the zygapophyses like those of the lumbars and 
only imperfectly so, and the metapophyses are very prominent. 
There must have been at least four lumbar vertebrae (PI. XXIV, fig. 2) 
and in some individuals probably five ; the centra become broader and 
more depressed posteriorly and that of the last lumbar is very broad and 
low and is traversed by a pair of vascular canals, which open upon the 
