160 Sir Everard Home on the canal for containing 
in what manner that is done, and in this as well as in many 
other particulars, the skeleton differs from that of other 
animals. 
In all the lizard tribe, the vertebras, like those of other 
quadrupeds, consist of a body, two transverse, and one spi- 
nous process, surrounding a central hole, which forms the 
spinal canal ; the vertebras of the different tribes of Proteus 
have the same structure, as is shown in a Plate annexed to 
my former Paper upon that subject. In cartilaginous fishes, 
the mechanism is very different ; the bodies of the vertebras, 
properly speaking, form the spinal column ; and the canal 
for the spinal marrow r is surrounded by separate cartilages, 
united to the vertebrae, by projections fitted to corresponding 
notches in the side of each vertebra. Through the whole 
length of the tail, a similar canal is formed upon the under 
edge of the vertebrae, in which are contained and defended 
from injury, the great artery and vein. In the fossil remains 
of the shark tribe, the bodies of these vertebrae are very fre- 
quently met with, and have hitherto been readily distin- 
guished from all others by these peculiarities. 
The structure of the vertebrae of the Proteosaurus, is some- 
thing intermediate between the two kinds which I have de- 
scribed. It is composed of bone, as in the lizard tribe, but 
consists of a body only, behind which is a canal for the spinal 
marrow, and the spinous process with two lateral branches 
which belong to it, quite distinct from the body of the ver- 
tebra, and between these there is no bony union, but a species 
of joint peculiar to themselves ; the canal thus formed, de- 
fends the spinal marrow from injury, equally as when the 
whole vertebra is in one piece ; it is unusually small. The 
