PRINCIPLES OF VERTEBRATE MORPHOLOGY 



17 



of the entire body, terminating in powerful propulsion by tlie tail 

 fin; secondly, by supplementary action of the four lateral fins; 



Fig. 4. — Three aquatic types of vertebrate, to illustrate convergent adaptation 

 of three wholly unrelated forms for marine life. All three show the fusiform body, 

 median and paired fins, though the skeletal structures are radically different. 

 A, shark (Pisces) ; B, ichthyosaur (Reptilia) ; C, porpoise (Mammalia) . (After 

 Osborn's " Origin and Evolution of Life " [Charles Scribner's Sons].) 



third, by the horizontal steering of the body by the median sys- 

 tem of fins." 



