LIFE 



379 



33 



Trilobitcs reached their climax in the Ordovician period, more 

 than half of all the known genera being represented in the Ordovi- 

 cian system. But few of them lived over from the Cambrian. In 

 thr next period the numbers fell off a full half, and the decline con- 

 tinued until the tribe became extinct. The general aspect of the 

 ilobites at the high tide of their career is fairly illustrated in Fig. 

 35. There was little or no increase in average size, as compared 

 with their Cambrian forbears. Some individuals had a length of 

 1 8 inches; but this size was equaled and even surpassed by some of 

 eir predecessors. 



Cephalopods. The largest, most powerful, and perhaps most 

 redaceous forms of Ordovician life, seem to have developed into 



O 



s 



D 



Pe T 



Fig. 336. The two upper curves represent the history of the trilobites according 

 to genera, the full line indicating the total number of genera, and the dotted line 



" e number of new genera introduced. The two lower curves present the same data 



>r the families of the trilobites. Data for families from Beecher in the Zittel- 

 Eastman text-book of Paleontology, Vol. I. Data for genera, somewhat incom- 



Ku . from Zittel's " Handbuch der Palaontologie." , Cambrian; O, Ordovician; 



', Silurian, etc., Pe, Permian, and T, Trias. 



prominence suddenly. Unless the fishes, of which little is known, 

 contested their supremacy, they were doubtless the undisputed 

 masters of the sea. Their general aspect is seen in Fig. 337. The 

 dominant type, as well as the most primitive one, had long, straight, 

 gently tapering shells (Fig. 337, c and/) divided into chambers by 



flane partitions (septa). Even in the Ordovician period there was 

 wide departure from this ideal simplicity. There were curved 



