A VIEW IN PERSPECTIVE. 



127 



Fig. 17. Some groups have appeared in greater or 

 less abundance in the Silurian (2), have rapidly cul- 

 minated, and then slowly dwindled away, but have 

 never become entirely extinct, even up to the present 

 day (see Fig. 17). The crinoids form our best illus- 

 tration. They were present in abundance early in 

 the Silurian (2) rocks, they rapidly expanded, reached 



FIG. 17. 



their culmination in the Carboniferous (4), and have 

 since that time been constantly diminishing in num- 

 bers. They are not yet extinct, but only eight genera 

 of this once predominant type are known to exist, 

 and most of these are confined to the depths of the 

 sea. The Brachiopoda also were so abundant in the 

 Silurian (2) age that they may be called the charac- 

 teristic animals of that era. Immediately after the 



