468 F. VON HUENE DEYELOPMEKT OF THE ICHTHYOPTERYGIA 



fin, being immobile, together with the dorsal fin, functions as a rudder.. 

 The wriggling lower wing of the caudal fin acts as a propeller. In the 

 later Mesozoic ichthyosaurs the trunk becomes relatively longer and the 

 anterior half of the tail is shortened. Therefore the whole body becomes 

 more torpedo-like and better adapted for swimming, as in the modern 

 XipJiias gladius. 



