CHAPTER XXVIII 



MESOZOIC PERIODS — TRIASSIC 



The Mesozoic era, so far as we can judge, seems to have been 

 much shorter than the Palaeozoic ; in North America Mesozoic 

 rocks are very much more important and widely spread in the 

 western half of the continent than in the eastern. The latter 

 region was, in a measure, completed by the Appalachian revolu- 

 tion, and subsequent growth consisted merely in the successive 

 addition of narrow strips to the coast-line, but in the West many 

 great changes were required to bring the land to its present condi- 

 tion. 



The life of the Mesozoic constitutes a very distinctly marked 

 assemblage of types, differing both from their predecessors of the 

 Palaeozoic and their successors of the Cenozoic. In the course of 

 the era the Plants and marine Invertebrates attained substantially 

 their modern condition, though the Vertebrates remain through- 

 out the era very different from later ones. Even in the Verte- 

 brates, however, the beginnings of the newer order of things may 

 be traced. In the earlier two periods, the Triassic and Jurassic, 

 vegetation is almost confined to the groups of Ferns, Cycads, and 

 Co?iifers, but with the Cretaceous come in the Angiosperms, both 

 Monocotyledons and Dicotyledons, and since then the changes have 

 been merely in matters of detail. 



With few exceptions, the ancient Tetracoralla have all disap- 

 peared, and the modern Hexacoralla take their place. The 

 Echinoderms are all markedly different from the Palaeozoic types. 

 The Cystids and Blastoids have died out, and the Crinoids have 

 been revolutionized, the Palczocrinoidea being replaced by the 

 Neocrinoidea. Likewise the modern sea-urchins, Euechinoidea, 

 replace the ancient Palczoechinoidea, and many Mesozoic genera 



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