PLESIOSAURTTS. 161 



the habits of the Plesiosaurus a probable cause for this ex- 

 traordinary deviation from the normal character of the 

 Lizards. 



Bach and Tail. 



The vertebra? of the back were not disposed in hollow 

 cones, like those of fishes, but presented to each other near- 

 ly 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 pe- 

 culiar 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 wath the structure of 

 the other parts of the Plesiosaurus, the combination of 

 strength, rather than of speed with flexibility, was more im- 

 portant. 



The tail, being comparatively short, could not have been 

 used like the tail of fishes, as an instrument of rapid impul- 

 sion 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 de- 

 scending 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 vertebras 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. 



14^ 



