524 



HISTORICAL GEOLOGY 



from the shore. Consequently, the Foraminifera which throve in 

 great abundance near the shores as well as in the waters of the deep 

 seas, as they do to-day, were not upon their death covered with 

 clastic sediments, but in the course of time built up thick deposits 

 of lime, composed largely of their own remains. The genera and 

 species of Foraminifera have generally, as might be expected from 

 their low organization, a long range in time : some of the species which 

 occur in the Paleozoic are still living. 



Flint nodules, varying in shape and ranging in size from that of a 

 walnut to two feet in length, are of common occurrence in certain 

 portions of many chalk beds. They are composed largely of siliceous 

 protozoa (Radiolaria), sponge spicules, and silica that has no or- 

 ganic form. These flint nodules were probably formed by concre- 

 tionary action, the silica scattered rather uniformly throughout the 

 deposits being brought together to form masses of varying size. 



Sponges. — Sponges appeared in the Triassic of Europe in small 

 numbers and became so numerous in certain localities during the 

 Jurassic as to form thick strata with their remains. They were still 

 more abundant in the Cretaceous of Europe, though not common in 

 America. 



Corals. — The Paleozoic corals (Tetracoralla) did not immediately 

 give place to the modern type (Hexacoralla, Fig. 487 A, B) 9 but a few 

 lingered for a short time in the Triassic. Be- 

 fore the close of that period, the new type 

 (Hexacoralla) became so thoroughly established 

 as to build coral reefs where conditions were 

 favorable. It was not, however, until the 



Jurassic that ex- 

 tensive reefs 

 were formed by 

 their remains. 

 The general ap- 

 pearance of the 

 Cretaceous cor- 

 als is not unlike 

 that of the corals 

 of to-day. 

 ■This class (Fig. 488 A, B) was rare both near the be- 

 with some exceptions (Uintacrinus of the Niobrara 

 Chalk of Kansas), near the close of the Mesozoic, but became abun* 



FlG. 487. — Mesozoic corals: A, Thecosmilia trichotoma 

 (Triassic and Jurassic) ; B> Thamnastrcea prolifera (Jurassic). 



Crinoids. - 

 ginning and 



