The Era of Iiivertehrates 



graptolites illustrates these points very well ; 



this subclass of small animals related to the 



Jelly-fishes began its career in the Cambrian, 



reached its highest 



point of development 



in the Ordovician, and 



disaj^jDeared entirely 



during the Devonian 



period, having run its 



entire cycle in this 



comparatively short 



space of time. 



Sea -lilies, or cri- 

 noids, have increased, for the simpler forms of 

 the group are at their maximum and the more 

 highly developed and beautiful species have 

 made their appearance, although their day of 

 supremacy is still to come. Corals, too, have 

 increased in numbers and species, but they 

 have not as yet begun to play their part in 

 the scheme of life by the building of reefs to 

 deflect tides and currents and form harbors of 

 refuge for many aquatic animals. More than 

 this, in the Water Lime group of rocks a scor- 



71 



Graptolites, 

 Dendrograptus hallian us. 



