494 MESOZOIC TIME — REPTILIAN AGE. 



were in progress along its more southern borders and through its 

 interior seas. 



In America there are but few distinct formations, the Triassic and 

 Jurassic making seemingly one continued series, and the Creta- 

 ceous another with three or four subordinate divisions. In Europe 

 the number of epochal changes, or abrupt transitions in the rocks, 

 is large, — much more so than in the Carboniferous age. 



In America there are no limestones, and therefore no evidences 

 of clear interior seas, except in the closing epoch of the Cretaceous 

 in Texas, and some thin interpolations in the earlier formations. 

 In Europe the Lias, and a large part of the Oolite and Chalk, are 

 limestone formations. 



The facts indicate great simplicity and but narrow limits in the 

 oscillations of North America, and remarkable complexity and 

 diversity of extent in those of Europe. 



III. Life. 



1. Decline in Palaeozoic Features. 

 Decline in Carboniferous genera of plants. — Of the genus Calamites 

 about 50 Carboniferous species have been described, only 3 or 4 

 Triassic, 2 Jurassic, and none of later periods. The genera of ferns 

 Pecopteris, Neuropteris, Sphenopteris, and Cyclopteris are continued in 

 the Mesozoic ; but only one species in all — a Pecopteris — has been 

 found in the Cretaceous beds. 



Pecopteris had 50 species in the Carboniferous age, 5 or 6 in the Triassic 

 period, the same in each the Jurassic and the Cretaceous; Neuropteris, 50 Car- 

 boniferous, 8 Triassic, 6 Jurassic, none Cretaceous; Sphenopteris, 75 Carboni- 

 ferous, 20 Triassic and Jurassic, none Cretaceous ; Cyclopteris, 34 Carboniferous, 

 4 Triassic, Jurassic none, Cretaceous none. 



Decline in Crinoids. — There were over 500 species of Palaeozoic 

 Crinoids ; of Jurassic, about 75 ; of Cretaceous, 15. Considering 

 the time-ratio for the Palaeozoic and Mesozoic, 3| : 1, these numbers 

 indicate an approximate equality between the eras. But it is still 

 true that the Mesozoic is much less prolific in individuals. 



Decline in Brachiopods. — The number of known Palaeozoic spe- 

 cies is about 700 ; of Triassic and Jurassic, 150 ; of Cretaceous, 

 about the same. These numbers do not exhibit the fact of the 

 decline. But it is strikingly seen in this : that the Brachiopods 

 were one-third the whole number of Mollusks in the Palaeozoic, and 

 only one-eleventh in the Mesozoic. Moreover, the Mesozoic species 

 belong extensively to the Terebratula family, which is eminently a 



