48 THE LESSON OF EVOLUTION 



tance in the Triassic period, attained their maximum 

 in the Jurassic, and still remained numerous in the 

 Cretaceous. Generally they were rapidly changing 

 animals, the genera, and even the families, being, 

 for the most part, different in each of the three 

 periods; while, during the time of their greatest 

 development, several species had such short dura- 

 tion that they are used to discriminate thin zones 

 in the Jurassic rocks. So rapidly did they change 

 that, in many species, we find the form of ornamen- 

 tation altering during growth, that of the young 

 shell preserved on the inner whorls gradually 

 changing into a different pattern on the outer whorls. 

 But the genera Phylloceras and Lytoceras form ex- 

 ceptions to the rule ; for they existed with very little 

 alteration from the end of the Trias to the upper 

 Cretaceous. Perhaps the most remarkable thing 

 about the Ammonites is their sudden and complete 

 disappearance, together with the Belemnites, all 

 over the world at the end of the Cretaceous period. 

 They had declined during the Cretaceous with the 

 increase of predaceous sharks, and it is possible that, 

 encumbered by their shells, which were too fragile 

 to be any protection, they could not escape from 

 these new enemies. 



The fishes were chiefly Actinopterygii. In the 

 early Mesozoic they had imperfectly ossified skele- 

 tons, enamelled scales, and heterocercal tails; but 

 these gradually passed into forms like modern fishes, 

 with a well ossified skeleton, bony scales, and homo- 

 cereal tails. In the Jurassic period sharks appeared 

 with sharp, pointed teeth in their jaws ; but they 



