PLESIOSAURUS. 163 



This hypothesis of Cuvier is but conjectural, respecting 

 the power of the Plesiosaurus to change the colour of its 

 skin ; and to the unexperienced in comparative anatomy, 

 it may seem equally conjectural, to deduce any other con- 

 clusions respecting such perishable organs as the lungs, 

 from the discovery of peculiar contrivances, and unusual 

 apparatus in the ribs ; yet we argue on similar grounds, 

 when from the form and capabilities of these fossil ribs, we 

 infer that they were connected, as in the Chameleon, with 

 vast and unusual powers of expansion and contraction in 

 the lungs; and when, on finding the ribs and wood-work 

 of a worn-out bellows, near the ruins of a blacksmith's 

 forge, we conclude that these more enduring parts of the 

 frame of this, instrument, have been connected with a pro- 

 portionable expansion of leather. 



The compound character of the ribs, probably also gave 

 to the Plesiosaurus the same power of compressing air 

 within its lungs, and in that state taking it to the bottom,, 

 which we have coufsidered as resulting from the structure of 

 the sterno -costal apparatus of the Ichthyosauri. 



Extremities.* 



As the Plesiosaurus breathed air, and was therefore 

 obliged to rise often to the surface for inspiration, this ne- 

 cessity was met by an apparatus in the chest and pelvis, 

 and in the bones of the arms and legs, enabling it to ascend 

 and descend in the water after the manner of the Ich- 

 thyosauri and Catecea ; accordingly the legs were converted 

 into paddles, longer and more powerful than those of the 

 Ichthyosaurus, thus compensating for the comparatively 

 small assistance which it could have derived from its tail.f 



* See PI. 16, 17, 19. 



t The number of joints representing the phalanges of the fingers and 

 toes exceeds that in the Lizards and Birds, and also in uU Mammalia; 

 excepting the Whales, some of which present a similar increase of num- 



