74 THE DOCTRINE OF DESCENT. 



And SO it proves. The Placoids and Ganoids hitherto 

 predominating in the ocean almost without a foe, now 

 found overwhelming enemies in the true sea-lizards, or 

 Enaliosaurians, especially the Ichthyosaura and Plesio- 

 saura. The head is like a lizard or a crocodile, the ver- 

 tebral column fish-like, and, as Gegenbauer has shown, 

 the extremities also recall the simpler fins of sharks. 

 Their coprolites likewise allow us to infer with full 

 certainty, a very peculiar construction of the middle 

 portion of the intestinal canal. They possessed a spiral 

 intestine like that of the sharks and their congeners. 

 These animals are therefore noteworthy, not only on ac- 

 count of their striking external appearance and the part 

 they play in nature's household, but, like the Labyrintho- 

 don, as intermediate and connecting forms of reptiles and 

 of fish. 



Besides these animals, a conspicuous _ group of the 

 marine fauna is that of the Ammonites, which occur 

 in masses, and next to the Nautili constitute the chief 

 form of the ancient Cephalopoda, the study of which 

 has recently promised to contribute essentially to the 

 decision of the most important points in our science. 

 In combination with them, the Belemnites abound with 

 their multitudinous species, originating in the Trias. 

 They are proved to be the predecessors of dibranchiate 

 Cephalopoda, which now predominate. On the chalk 

 plains of Eichstadt and Solnhofen, belonging to the 

 White Jura, are also preserved impressions, resembling 

 drawings, of Medusae, which show that even at that time 

 this class had reached the state in which it still exists. 



The terrestrial fauna of the Jurassic period is like- 

 wise enriched by new forms and groups. We find the 



