THE ORDOVICIAN PERIOD 427 



sandstones of New York, Pennsylvania, and West Virginia, however, 

 have furnished the richest oil-bearing strata of the eastern United 

 States. 



Life of the Ordovician 



The life of the Ordovician differed from that of the Cambrian in the 

 abundance of certain classes which were rare in the latter, and in the 

 higher level of development in many cases. Graptolites, although 

 rare in the Cambrian, attained their greatest abundance in the Ordo- 

 vician. The primitive corals of the Cambrian were followed by well- 

 developed forms; the cephalopods became the largest animals of the 

 period ; gastropods were much more modern in appearance ; and 

 brachiopods show a great increase in variety and abundance. 



Protozoa 



Siliceous protozoa (Radiolaria) are found in the Ordovician strata 

 of some regions in sufficient numbers to show that they were abun- 

 dant in the seas of that period. 



Ccelenterata 



Sponges are well represented by forms that secrete siliceous skeletons 

 (Fig. 396 A, B), and some of them attained a diameter of a foot or 





1 i m 



--"^r--" B 



Fig. 396. — Ordovician sponges: A, Brachiospongia digitata; B, Receptaculites 



ohioensis. 



more. Certain beds of the Ordovician (Chazy), of New York, are 

 composed almost entirely of sponges. 



Graptolites. — This class (Fig. 397 A-K) can be traced from its 

 earliest appearance to its final extinction, through all the stages of 

 development, and is consequently well-adapted to illustrate some 

 principles of evolution. 



