12 A BOOK OR WHALES 



fringes, such as certain aquatic tortoises have at the 

 present day. But no doubt can exist as to the 

 possession of lungs. Therefore the extinct "fish 

 lizard" also must have come to the surface of the 

 Cretaceous seas to "spout." 



But its tail is fish-like in its verticalness ; and, if we 

 are to suppose that it resembled the whale in its 

 diving and ascending to the surface, it is difficult to 

 understand how it is that the tail is not made after 

 the best pattern for affecting such movements. As 

 a matter of fact it seems, according to Professor 

 Ahlborn, that the Ichthyosaurus tail was suitable to 

 a life of constant interchange between air and water, 

 but in a different way from that of the whale. Dr. 

 Ahlborn has remarked in a recent and highly interest- 

 ing paper 4 that the Ichthyosaurus and the shark 

 stand in regard to their tail at the two opposite poles 

 of aquatic creatures. They both possess what is 

 termed in the fish a " heterocercal " tail. This kind 

 of tail is marked by the fact that the backbone is 

 continued into the edge of the actual tail fin, the 

 upper edge in the case of the shark, the lower edge in 

 the reptile ; so that in both cases the bulk of the 

 actual fin itself lies either above or below the 

 strengthening bar of bones and cartilages. It is 

 suggested that the "epibaty" or " hypobaty " of the 

 tail corresponds to a different function in the two 

 cases. 



In the shark the movements of the body generally 



"Ueber die Bedeutung der Heterocerkie," etc., Zeitschr. wiss. Zoo!., 

 Ixi., p. i. 



