72 



HISTORICAL GEOLOGY 



marine forms, and it is an interesting and important fact that such 

 very simple types have persisted throughout all of geologic time 

 from the Cambrian to the present, while profound evolutionary 

 changes were taking place in the animal kingdom. Radiolarians 

 are not known as fossils. Many Protozoans doubtless existed, but 

 very few secreted shells, and hence not many species could have 

 been preserved as fossils. 



Fig. 32 

 Calcareous Algae, Cryptozodn proliferum, forming a reef 

 in Upper Cambrian limestone near Saratoga Springs, 

 New York. (After H. P. Cushing, N. Y . State Mus. 

 Bid. 169.) 



Porif ers. — True Sponges (Fig. 33) were fairly abundant 

 throughout the period, their siliceous spicules being especially 

 common as fossils. 



Ccelenterates. — Hydrozoans were represented by both the 

 so-called "Jelly-fishes" and the Graptolites. The finding of many 

 recognizable casts and impressions of Jelly-fishes (Fig. 34), which 

 consist wholly of soft parts, is a remarkable freak of fossil preser- 

 vation. Graptolites (Fig. 49) were common, especially in the later 

 Cambrian. These were slender, plume-like, delicate forms consist- 

 ing of colonies of cells. They were pelagic or free to float in the 

 open sea. One genus of Graptolites, confined to a horizon near 



