352 



HISTORICAL GEOLOHV. 



least, to a lost interval between the Jura-Trias and the 

 Cretaceous. Of the four hundred and sixty species of 

 plants found in the Middle Cretaceous of the West, four 

 hundred are dicotyls. Nearly all the genera of common 

 trees are represented, although, of course, the species are 

 extinct. There were then, as now, oaks, maples, willows, 

 sassafras, dogwoods, hickory, beech, poplar, tulip-tree 

 (Liriodendron), walnut, sycamore, sweet-gum (Liquid- 

 ambar), laurels, myrtles, etc. A few of these are given 

 in Figs. 305-307. 



Animals. 



Protozoa. Though these are found in nearly all the 

 strata heretofore described, we have usually neglected 



FIG. 308. F.G. 300. FIG. 310. 



TIGS. 308-310. Foranrinifera of chalk, magnified : 308. Flabellina rugosa. 309. 

 Lituola nautiloides. 310. ChryBalidina gradata. (After D'Orbigny.) 



them, because they are inconspicuous. But here in the 



Cretaceous they 

 are so abundant 

 that they demand 

 attention. Chalk, 

 as already said, is 



aminifers (Figs. 



FIG. 311. Echinoicls of the Cretaceous of Europe : OAO o-in\ j 



Galerites albogalerns. d 8 - d 1 ) , and 



sponges are also 



extremely abundant. Of the former, some are identical 

 with living species. 



