LIFE ON THE EARTH. 35 



flexible tail of the Crayfish (Palinurus), distinct from 

 that of the true Astacidse in those remote ages as 

 it is still found to be in the corresponding groups 

 living together in the ocean. Equally curious to 

 notice in the Ichthyosaurus and Teleosaurus of the 

 Lias the same principles of watery propulsion, by the 

 tail and lateral paddles, as in the Fish, Turtles and 

 Alligators of the present period. One general idea 

 runs through the whole series, the propulsive power 

 is worked by a system of leverage placed more or less 

 retrally, aided by a directing power placed forward. 

 If Plesiosaurus appear to be an exception, the ano- 

 maly may be regarded as compensated by the exten- 

 sion of the neck, for thus in fact the dynamic centre 

 is really retral, as to the general figure, while the 

 approximation of the paddles gives remarkable power 

 of quick turning, suitable to its way of life. 



Diving. The same backward position of the 

 propulsive organs is even more remarkable in the 

 case of the diving birds, such as Colymbus, Uria, 

 Aptenodytes, whose general form resembles that 

 given by mechanicians to ships constructed for swift 

 motion, and suggests to every observer the idea of 

 the solid of least resistance in water. The same 

 peculiarities appear in a different manner in the very 

 different group of diving insects, such as Dytiscus, 

 Colymbetes, and Hydrophilus, and may be found 



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