THE GREAT SEA-LIZARDS AAW 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 vertebrae 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. 



