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, and 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 ; ou 

 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 3^ inches in 

 breadth forwards, gradually diminishing 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 vertebra? ; 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 X, for the ribs which unite by 

 them with the spinal canal. 



The third part of the spinal canal unites with the end of the 

 •econd 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 beginning 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 



