258 GEOLOGY. 



furnished a large portion of the flint for the nodules in 

 the chalk. The sponges abounded at that period, and 

 had an agency in regard to silicious matter similar to 

 that which corals have had in relation to limestone, and, 

 to some extent at least, they have that agency still. 



363. Animals of the Cretaceous Period. I have al- 

 ready spoken of the Foraminifera and the Sponges. The 

 corals were not so abundant as in the Jurassic period. 

 The sea-urchins were very numerous, and the preserva- 

 tion of their beautiful remains is one of the marked fea- 

 tures of the period of the Chalk. The Crinoids, or En- 

 crinites, were decidedly on the wane. The Crustaceans 

 were on the increase, and approached in character to the 

 crabs and lobsters of the present day. The fishes ex- 

 hibited a great change. Though some of the placoids 

 and ganoids of former ages still remained, the ctenoids 

 and cycloids, which are now the prevailing orders, first 

 appeared in the Cretaceous age. The great reptiles of 

 the Jurassic period were passing away, for this was the 

 concluding period of the Reptilian age. The higher 

 mollusks appeared in great profusion. There was a re- 

 markable change in the forms of that great tribe of mol- 

 lusks, the Ammonites. Before this the coil of their cham- 

 bered cells was close and on one plane, as seen in Fig. 

 147 ; but now the coil was more or less open in many 

 of the species, sometimes with fantastic variations, or it 

 was spiral. Examples are given in Fig. 158, where we 

 have, 1. Ancyloceras, incurved like a crosier. 2. Scaph- 

 ites, curved like the prow of a skiff. 3. Crioceras, curled 

 like a ram's-horn. 4. Samites, bent like a hook ; and, 5, 

 Turrilites, running in a spiral round a straight axis. 



364. Uplifts at the Close of the Reptilian Age. While 

 there were some disturbances during the progress of this 

 age, there were great and extensive ones at its close, in 

 the interval of passage from Mesozoic to Cainozoic time, 

 just as there were at the close of Palaeozoic time, as no- 

 ticed in 337. Some of the great chains of mountains 



