MESOZOIC ERA. AGE OF REPTILES. 325 



though very distinct from the Cretaceous ; so that, studied 

 in America alone, it would be most natural to divide 

 the whole age into two periods: 1. Jura-Trias ; and, 

 2. Cretaceous. 



Again, the Jura-Trias is much poorer in fossils in this 

 country than in Europe ; so that, if we treated of American 

 strata alone, we should give but a very imperfect picture 

 of the times. Therefore, our plan will be to give a brief 

 sketch of the Trias, and then of the Juras, taking illus- 

 trations chiefly from foreign sources, and then a sketch 

 of the Jura-Trias in America. The Cretaceous can be 

 fully illustrated from American strata. 



SECTION I. TEIASSIO PERIOD. 



As already stated, the lowest Mesozoic (Triassic) is 

 always, or nearly always, unconformable with the Coal. 

 The line of break may be between the Triassic and the 

 Permian, but more commonly between the Permian and 

 the Coal. But the fossils of the Triassic are always very 

 different from those of the Permian. The break in the 

 life system is always greatest here. We will neglect the 

 subdivisions, and take up all together. 



Life System, Although the revolution which closed 

 the Paleozoic is passed, and comparative quiet again re- 

 stored, yet it took some time for the old fullness of life 

 to recover itself. Mesozoic life, therefore, is compara- 

 tively poor in the Triassic compared with the Jurassic 

 and Cretaceous. We will, therefore, touch very briefly 

 on Triassic life. 



The Change. The most striking fact is the sweep- 

 ing change in life-forms. All the old style corals are 

 replaced by new style ; all the armless crinoids (Blastids 

 and Cystids), the square-shouldered brachiopods, the 

 orthoceratites and trilobites, the lepidodendrids, sigil- 

 larids, and calamites in a word, all that we found most 



