PLESIOSAUUrS. 
207 
Sack and Tail. 
The vertebree of the back were not disposed in 
hollow cones, like those of fishes, but presented 
to each other nearly flat surfaces, giving to the 
column a stability, like that which exists in the 
back of terrestrial quadrupeds. The articulat- 
ing processes, also, were locked into one another 
in such manner as to give strength, rather than 
that peculiar kind of flexibility, which admitted 
of the same quick progressive motion in the 
Ichthyosauri that we find in fishes : but as rapid 
motion was incompatible with the structure of 
the other parts of the Plesiosaurus, the combi- 
nation of strength, rather than of speed with 
flexibility, was more impoi’tant. 
The tail, being comparatively short, could not 
have been used like the tail of fishes, as an 
instrument of rapid impulsion in a forward direc- 
tion ; but was probably employed more as a 
rudder, to steer the animal when swimming on 
the surface, or to elevate or depress it in ascend- 
ing and descending through the water. The 
same consequence as to slowness of motion 
ivoidd follow from the elongation of the neck, 
to so great a distance in front of the anterior 
paddles. The total number of vertebrae in the 
entire column was about ninety. From all these 
Circumstances we may infer that this animal, 
although of considerable size, had to seek its food. 
