200 THE STORY OF REPTILE LIFE. 



balancers. These resemble the single back fin 

 of certain whales, in that they were unsupported 

 by a bony skeleton as in fishes. Then the tail 

 differed in a very important particular, in a way, 

 indeed, which makes it unique among animals, 

 inasmuch as it turned sharply downwards. In 

 the whales and other allies this organ is 

 perfectly straight, just as in the primitive fishes. 

 In the "Story of Fish-life," it may be remem- 

 bered, it was pointed out that these primitive 

 types were succeeded by others in which the 

 tail was bent directly upwards, and that finally 

 this upturned portion was gradually suppressed. 

 It is with those fishes in which the upturned 

 portion of the tail is still retained that we must 

 compare the tail of the Ichthyosaurus. This 

 portion of the tail then supported a tail Jin which 

 was made up of a number of rods supporting a 

 membrane. In the Ichthyosaurus the correspond- 

 ingly downwardly directed tail also supported a 

 fin, but this again resembles the back fin in that 

 it was entirely fleshy and had no supports. Inas- 

 much as this fin was vertical in position, it resem- 

 bled the fin of fishes, and differed conspicuously 

 from the huge tail fin of the whales and their 

 allies, which is horizontal in position. This is a 

 point of some importance. The horizontal posi- 

 tion of the tail fin is peculiar to the aquatic 

 mammalia, such as the whales and their imme- 

 diate allies, and the manatees and dugongs, and 

 has apparently been evolved to suit the need of 

 these creatures for a constant and frequent 

 supply of fresh air ; the up and down motion of 

 the tail driving the animal either up to the 



