Life in the Ancient Seas 313 



protozoans far outranked alT other forms; and in the 

 various localities in which they thrived, each of these 

 groups built up huge masses of limestone. Here, for 

 the first time in the history of our earth's crust, we find 

 the shell-secreting protozoans (Foraminifera) taking 

 a conspicuous place among marine animals. Dense sub- 

 marine groves of crinoids flourished then as probably 

 they never did before, and their remains to-day are 

 consolidated into deep beds of rock. Sponges grew in 

 the greatest profusion. The brachiopods reached a 



PTERICHTHYS; AN ANCIENT FISH. 



great size and their shells became exceedingly thick. 

 Hardly less in importance were the mollusks, the uni- 

 valve and bivalve types in particular growing increas- 

 ingly abundant. Crustaceans of the higher types now 

 began to assume some prominence, but they were for 

 the greater part represented by the shrimps and 

 prawns. Throughout this period until almost to the 

 very close the sharks were in undisputed possession of 

 their realm. 



And then It was that those monstrous reptilian crea- 

 tures from the land Invaded the sea. Like the whales, 

 their structure was modified to suit their aquatic life. 



