THE GREAT SEA-LIZARDS AND THEIR ALLIES. 57 
Great credit is due to Mr. Alfred N. Leeds, of Eyebury, who has 
disinterred the separate bones of many distinct skeletons of 
Plesiosaurs from Oxford Clay strata near Peterborough. 
It will be remembered that the long and powerful tail of the 
fish-lizard was its principal organ of propulsion through the 
water ; and that, consequently, the paddles only played a 
secondary part. They were small, but amply large enough for 
the work they had to perform. But our long-necked sea-lizards 
possessed very short tails. What, then, was the consequence ? 
Obviously that the paddles had all the more work to do. They 
were the chief swimming organs. The vertebrse of this short tail 
show that it probably was highly flexible, and could move rapidly 
from side to side ; but, for all that, its use as a propeller would 
not be of much importance. We see now why the paddles are 
so long and powerful, like two pairs of great oars, one pair on 
each side of the body. In a fossil skeleton you will notice the 
flattened shape of the arm-bone (or humerus), and of the thigh- 
bone (or femur). This gave breadth to the paddles, and made 
them more efficient as swimming organs. They give no indica- 
tion of having carried even such imperfect claws as those of 
turtles and seals, and therefore we may conclude that the Plesio- 
saur was far more at home in the water than on land, and it 
seems probable that progression on land was impossible. 
The tail was probably useful as a rudder, to steer the animal 
when swimming on the surface, and to elevate or depress it in 
ascending and descending through the water. Like the fish- 
lizard, this creature was an air-breather, and therefore was obliged 
occasionally to visit the surface for fresh supplies of air. But 
probably it possessed the power ot compressing air within its 
lungs, so that the frequency of its visits to the surface would not 
be very great. 
From the long neck and head, situated so far away from the 
paddles, as well as for other reasons, it may be concluded that 
this creature was a rapid swimmer, as was the Ichthyosaurus. 
