THE ORDOVICIAN PERIOD 



525 



numbers fell off a full half, and this decline continued until the 

 tribe became extinct. The general aspect of the trilobites at the 

 high tide of their career is fairly illustrated in Fig. 386. Their eyes 

 were, as a rule, more prominent and better developed than those 

 of the Cambrian species. There was little or no increase in average 

 size. Some individuals reached a length of 18 inches and ranked 



Fig. 387. Ordovician Cephalopoda: a, Poterioceras apertum Whiteaves; 

 b, Cyrtoceras neleus Hall; c, Orthoceras bilineatum Hall; d, Oncoceras 

 pandion Hall; e, Trocholites ammonius Conrad; /, Orthoceras sociale 



among the giants of the group, but this size was equaled and even 

 surpassed by some of their Cambrian forebears. 



Besides the trilobites, the crustaceans were represented by a 

 few inferior forms, such as ostracodes (Fig. 372) and cerripeds. 



The Cephalopods. The largest, most powerful, and perhaps 

 most predaceous of the known forms of Ordovician life were the 

 cephalopods, which seem to have developed into prominence with 

 extraordinary suddenness. Unless the fishes, of which little is 



