THE PROGRESS OF LIFE 81 



MESOZOIC LIFE 



In the Mesozoic era the chief thing we should 

 notice in the ocean is the great abundance of the 

 Ammonites. Beginning in the Deutozoic, they had 

 acquired considerable importance 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 duration 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 

 ornamentation 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 excep- 

 tions 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, encum- 

 bered 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 skeletons, 

 enamelled scales, and heterocercal tails ; but these 



