FOSSIL STAR FISHES AND SEA-URCHINS. 137 



not appear to have thriven well. Only two genera 

 are known, and these are represented by few species 

 during periods long enough to form strata thicker 

 than all the Secondary deposits taken together. But 

 when we come to the Secondary period, we find the 

 sea-urchins gaining ground. By-and-by, as in the 

 Chalk formation, they are wonderfully common, and 

 of multitudinous shapes and types. But by this time 



Fig. 115. Recent Ophiocoma (British seas). 



the Encrinites, which we have seen were so plentiful 

 on the floors of primaeval seas, had begun to decline. 

 Broadly, therefore, it may be stated that the sea- 

 urchins began to flourish just when the Encrinites 

 commenced to dwindle away. 



Fossil star-fishes are not as a rule abundant, 

 unless, perhaps, we except a particular stratum in the 

 Middle Lias, where they are in places so plentiful 



