CHAPTER XXIII 
LIFE DURING THE MESOZOIC AND CENOZOIC TIMES 
Mesozoic Life.—'The Mesozoic is divided into the 
Juratrias' and the Cretaceous. Although there are 
many differences in the life of the two parts of the 
Mesozoic, we may group the two periods together, in 
order to avoid repetition, and consider the history of 
development of the Juratrias and Cretaceous organisms 
at the same time. 
Among the marine invertebrates, change is noticeable 
in the decrease of the brachiopods, and the nearly com- 
plete extinction of the trilobites, as well as a decrease 
in numbers and variety of the crinoids. Parallel with 
this is an increase in the relative importance of the 
cephalopod and lamellibranch (the bivalve shells) groups 
of the Mollusca (Fig. 246). These attained wonderful 
variety of form, and peopled the seas in great abun- 
dance. The univalve shells (gasteropods) were also of 
greater importance during this time than in previous 
ages. It is among the cephalopods, however, that the 
1 This is also often divided into Jurassic and Triassic. 
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