210 MARINE SAURIANS. 
frame of this instrument, have been connected 
with a proportionable 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 
considered as resulting from the structure of 
the steno-costal apparatus of the Ichthyosauri. 
Extremities.^ 
As the Plesiosaurus breathed air, and was 
therefore obliged to rise often to the surface for 
inspiration, this necessity was met by an appa- 
ratus 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 
Ichthyosauri and Cetacea ; accordingly the legs 
were converted into paddles, longer and more 
powerful than those of the Ichthyosaurus, thus 
compensating for the comparatively small assist- 
ance which it could have derived from its tail.t 
Comparing these extremities with those of 
other vertebrated animals, we trace a regular 
* 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 all Mammalia, excepting the Whales, some of which present 
a similar increase of number to accommodate them to the corres- 
ponding office of a paddle. The mode of connection between 
the joints was (like that in the Whales,) by synchondrosis. 
The phalanges of the Plesiosaurus present a link, between the 
