74 THE DOCTRINE OF DESCENT. 



that at least a more copious preservation of organic 

 remains might be expected, for the ooHtic strata are 

 mostly depositions which have taken place without dis- 

 turbance. 



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 classes of sharks. 

 Their coprolites likewise allow us to infer with full cer- 

 tainty, a very peculiar construction of the middle portion 

 of the intestinal canal. They possessed a spiral intes- 

 tine like that of the sharks and their congeners. These 

 animals are therefore noteworthy, not only on account 

 of their striking external appearance and the part they 

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

 donton, as mongrel and connecting forms of reptiles 

 and of fish. 



In addition to these animals, we must distinguish 

 among the marine fauna the Ammonites, which now 

 appear in vast masses, and the Nautili, the second chief 

 form of the ancient Cephalopods, the study of which 

 has recently promised to contribute essentially to the 

 decision of the most important points in our science. 

 In combination Avith 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 



