in the Museum of Buenos Ayres. 93 
joined with the following bone of the vertebral canal by means of 
a very moveable articulation, so as to raise itself and retract itself 
on the neck. In the same manner the head moves itself by the 
operation of this trivertebral bone enabling it to enter more or 
less into the interior cavity of the shell, anal to protect itself in 
this retired position from the attacks of other animals, in the 
same manner as the existing Armadillo. If it were not for the 
presence of this bone, the head of the animal could not move 
itself out of the shell, or, at will, retract itself within. 
Mr. Huxley, who very well describes this bone as a piece 
composed of three vertebra, supposes that the great mobility of 
this bone was necessary for the respiratory motion of the thorax, 
the ribs not being sufficiently moveable at their articulation with 
the vertebral tube. I cannot participate in this opinion ; on 
the contrary, the true function of this trivertebral bone is to 
facilitate the motion of the head both forwards and backwards, 
as I have already explained. There is no difficulty in the mo- 
tion of the ribs on their articulations, notwithstanding that 
there is a remarkable difference between their conformation and 
that of other Mammalia, as is proved by the form of the arti- 
cular excavations at the side of the vertebral canal. 
The second part of the spinal canal is the longest; it is 
17 inches in length in its curvature, and about 34 inches in 
breadth forwards, gradually dimimishing to 2 inches. The 
anterior part is smooth, with the first and inferior of the three 
crests ascending gradually higher on the upper side, having ten 
round foramina on each side of the canal, for the transmission 
of the nerves of the myelon, which proves that this part of the 
canal was composed of eleven united vertebre ; but there is no 
vestige of external separation on the surface. Besides, we see 
on each side, outside the lateral crests, eleven articular impres- 
sions of a peculiar form, like a 30, for the ribs which unite by 
them with the spimal canal. 
The third part of the spinal canal unites with the end of the 
second part, not by an articulation, but by a cartilaginous and 
moveable juncture, which anatomists term synchondrosis. It is 
by this that the edges of the canals which are in contact slightly 
extend towards the sides. The part of the vertebral column 
which follows is also different in form, being a little broader, 
and, on the dorsal surface only, armed with a very high median 
crest, but without transverse processes on either side, in conse- 
quence of the absence of the ribs. 
But there is in the begmning of the canal, and on each side 
of this high dorsal crest, a process which projects forwards, and 
is in contact with the end of each lateral crest of the second part. 
In this process there is also the half of the articular excavation. 
