ICHTHYOPTEKYGIA 



201 



that the ancient animal must have resembled the whale tribe 

 and the fishes, in the absence 

 of any intervening constriction 

 or neck. 



This close approximation in 

 the Ichthyosaurs to the form of 

 the most strictly aquatic verte- 

 brate animals of the existing 

 creation, is accompanied by an 

 important modification of the 

 surfaces forming the joints of 

 the back-bone, each of which 

 surfaces are hollow, leading to 

 the inference that they were 

 originally connected together 

 by an elastic bag or " capsule" 

 filled with fluid — a structure 

 which prevails in the class of 

 fishes, in the Labyrinthodonts S . 

 and a few extinct aquatic rep- -< 

 tiles, in the existing perenni- 

 branchiate Batrachia, but not 

 in any of the whale or porpoise 

 tribe. 



With the above modifica- 

 tions of the head, trunk, and 

 limbs, in relation to swimming, 

 there co-exist corresponding 

 modifications of the tail. The 

 bones of this part are much 

 more numerous than in the 

 Plesiosaurs, and the entire tail 

 is consequently longer ; but it 

 dues not show any of those 



3> e 



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