214 THE HISTORY OF CEEATION. 



A very peculiar class of Vertebrate animals, long since 

 extinct, and whicli appears to have lived only during 

 the secondary epoch, is formed by the remarkable Sea- 

 dragons (Halisauria, or Enaliosauria, also called Nexipoda, 

 or Swimming-footed animals). These formidable animals 

 of prey inhabited the mesolithic oceans in great numbers, 

 and were of most peculiar forms, sometimes from thirty 

 to forty feet in length. From many and excellently pre- 

 served fossil remains and impressions, both of the entire 

 body of Sea-dragons as well as of single parts, we have 

 become very accurately acquainted with the structure of 

 their bodies. They are usually classed among Eeptiles, 

 whilst some anatomists have placed them in a much lower 

 rank, as directly allied to Fish. Gegenbaur's recently 

 published investigations, which place the structure of their 

 limbs in a true light, have led to the surprising conclusion 

 that the Sea-dragons form quite an isolated group, differ- 

 ing widely both from Reptiles and Amphibia as well as 

 from Fish. The skeleton of their four legs, which are 

 transformed into short, broad, paddling fins (like those of 

 fish and whales) furnishes us with a clear proof that the 

 Halisauria branched off" from the main-stock of Vertebrata at 

 an earlier period than the Amphibia. For Amphibia, as well 

 as the three higher classes of Vertebrata, are all derived 

 from a common primary form, which possessed oiAyJive toes 

 or fingers on each leg. But the Sea-dragons have (either 

 distinctly developed or in a rudimentary condition as 

 parts of the skeleton of the foot) more than five fingers, 

 as have also the Selachians or Primaeval fish. On the other 

 hand, they breathed air through lungs, like the Dipneusta, 

 although they always swam about in the sea. They, 



