586 
Mr. De la Beche and Mr, Conybeare on 
tebrae in three points of view; and fig. 11, which represents one 
farther in the series, to shew the change in the socket for the arti- 
culation of the lateral bones, which is much narrower, and is 
higher placed. 
We have as yet no means of even surmising the number of 
caudal vertebrae. 
The middle dorsals are considerably larger than those of the 
extremities of the column ; the measures of three from different 
parts of Col. Birch’s were : 
Diameter of Articulating Surface . Length of Side. 
Cervical .... 1 inch f 
Middle Dorsal . * 1 a 1A 
Caudal f 
The largest vertebra of Plesiosaurus we have yet seen was 
inches in diameter. 
On the whole, then, it appears from these remarks, that the ver- 
tebral column of the Plesiosaurus recedes from that of the Ichthyo- 
saurus in all the points in which the latter approaches to the fishy 
structure ; that the intervertebral substance must have been disposed 
much as in the Cetacea, and that on this account, as well as because 
the annular parts were firmly attached to the bodies, and therefore, 
by the locking into one another of their articulating processes, must 
have given a considerable degree of stability to the column, it must 
have possessed in a much less perfect manner the flexibility which 
facilitates the peculiar motion of the Ichthyosaurus and of fishes. 
But it will be presently seen that this was much less necessary to 
these animals, in as much as the structure of their extremities ren- 
dered them much more powerful instruments of progression. 
To the description of these extremities we next proceed, premising, 
that we have never seen the bone which we have called in the 
