378 



ORDOVICIAN PERIOD 



identified. One of these zones falls in the Cambrian, eight in the 

 Ordovician, and eleven in the Silurian. If these be taken as chrono- 

 logical bench-marks, the successive horizons of the different conti- 

 nents may be correlated accurately, and the progress of life in the 

 various quarters of the globe referred to a common standard. 



Marine life. The known faunas of the Ordovician consist 

 almost wholly of marine invertebrates, among which trilobites and 

 brachiopods hold the leading places. Brachiopods are most numer- 

 ous, trilobites highest in organization, and cephalopods most power- 

 ful; but the foreshado wings of a new dynasty are at hand, for re- 

 mains of fish are found in this system. 



Fig- 335- ORDOVICIAN TRILOBITES: a, Isotelus maximus Locke; b, Ceraurtts 

 pleurexanthemus Green; c, Trinucleus ornatus Sternberg; d, Pterygometopus calli- 

 cephalus (Hall); e, Proetus parviusculus Hall;/, Bumastus trentonensis (Emmons); 

 g, Calymene callicephala Green. 



