482 



MESOZOIC TIME. 



in progress along its more southern borders, and through its interior 

 seas. 



In Eastern America, and partly in Western, but few marked subdi- 

 visions of the formations can be made out, the Triassic and Jurassic 

 making seemingly one continued series, and the Cretaceous 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 Eastern America, there is but little limestone and little evidence 

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

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

 Western, there is less of limestone in the interior region than of frag- 

 mental rocks. In Europe, the Lias and a large part of the Oolite and 

 Chalk are limestone formations. 



The facts indicate great simplicity in the oscillations of North 

 America, and remarkable complexity and diversity of extent in those 

 of Europe. 



HI. Life. 



The following are some of the facts illustrating the general steps in 

 the progress of life during the Mesozoic era : — 



Plants. — Instead of forests of Conifers, Tree-ferns, and Ly co- 

 pods (Lepidodendra, etc.), as in the Carboniferous, there were forests 

 of Conifers, Tree-ferns, and Cycads ; and finally, in the Cretaceous 

 period, these forests included also Angiosperms and Palms. The type 

 of Cycads culminated in the Mesozoic, and afterward had relatively 

 lew representatives. 



Animals. — 1. Radiates. — Corals of the Paleozoic type, having 

 the parts multiples of four in number, the Cyathophylloids, were almost 

 wholly wanting, while those of the modern Astraea type, having the 

 rays a multiple of six, abounded. 



Among Echinoderms, Crinoids, so abundant and important as rock- 

 makers in the Paleozoic, were comparatively little numerous, while 

 Echinoids and Starfishes were common. 



Mollusks. — Brachiopods were vastly inferior in number of indi- 

 viduals to other higher species ; and the kinds which existed, as the 

 TerebratulcE, etc., were inferior to those of earlier time, the type of 

 Brachiopods having culminated in the Paleozoic era. 



Gasteropods were, to a considerable extent, of modern genera ; but, 

 unlike the moderns, the higher siphonated species (those having the 

 aperture of the shell beaked), as well as the siphonated Lamellibranchs, 

 were in the minority, these groups culminating in a subsequent era. 



Among modern genera, the following occur in the Jurassic : Rimula, Planorbis, Pain- 



