524 



GEOLOGY 



held the leading places. The brachiopods were most numerous, 

 the trilobites highest in organization, and the cephalopods most 

 powerful. But the foresh ado wings of a new dynasty were at hand, 

 for the remains of fish have been found in the strata of this system. 



Fig. 386. Ordovician Trilobites: a, Isotelus maximus Locke; b, Ceraurn* 

 pleurexanlhemus Green; c, Trinucleus ornatus Sternberg; d, Pterygome- 

 topus callicephalus (Hall); e, Proctus parviusculus Hall; /, Bumaetu* 

 trentonensis (Emmons) ; g, Calymene callicephala Green. 



Trilobites and other crustaceans. The rise and fall of the trilo- 

 bites is shown in the curve of Fig. 385. Their climax in the Ordovi- 

 cian appears to have been reached by a rapid ascent, which was 

 followed by a more gradual decline. More than half of all known 

 genera of trilobites are represented in the Ordovician system, but 

 only a few lived over from the Cambrian. In the next period the 



