the habits of the Plesiosaurus a probable cause for this extraordinary deviation from the normal character of the Lizards.
Back and Tail.
The vertebræ 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 articulating processes, also, were locked into one another in such a 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 combination of strength, rather than of speed with flexibility, was more important.
The tail being comparatively short, could not have been used like the tail of fishes, as an instrument of rapid impulsion in a forward direction; but was probably employed more as a rudder, to steer the animal when swimming on the surface, or to elevate or depress it in ascending and descending through the water. The same consequence as to slowness of motion would follow from the elongation of the neck to so great a distance in front of the anterior paddles. The total number of vertebræ 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, as well as its safety, chiefly by means of artifice and concealment.
