74 THE DOCTRINE OF DESCENT. 
that at least a more copious preservation of organic 
remains might be expected, for the oolitic 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 with them, the Belemnites abound with 
their miultitudinous 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 
