THE ORDOVICIAN PERIOD. 347 



order implies the presence of vegetation and of atmospheric conditions 

 suited to active, air-breathing organisms. 



The Main Record Marine. 



The marine invertebrates made up almost the whole of the recorded 

 fauna of the Ordovician, and among these the trilobites, the cephalo- 

 pods, and the brachiopods held the leading places. Of these, the 

 brachiopods were the most numerous, the trilobites the highest in 

 organization, and the cephalopods the most powerful. But the fore- 

 shado wings of a new dynasty were even then at hand. 



The appearance of the vertebrates. — In the Ordovician near Canyon 

 City, and at some other localities in Colorado, the remains of fish- 

 like organisms occur in great abundance, but, unfortunately, in poor 

 condition, associated with characteristic Ordovician invertebrates. 1 

 The fish relics are very fragmentary, but among them there are com- 

 plete dermal plates. Rarely, a few of these plates remain joined 

 to one another. There are also portions of the vertebral column. 

 The relics are sufficient to make it clear that fish-like forms lived in 

 some abundance, and that they were armed for defense. It is not 

 equally clear that they were armed for attack. 



The climax of the trilobites. — More than half of all the known 

 genera of trilobites, as listed by Zittel, were represented in Ordovician 

 times. Only a few of these came over from the Cambrian, while 

 the others make their first appearance in this period. In the next 

 period (Silurian), their numbers fell to one half, and in later periods 

 declined still further until they disappeared at the close of the Paleozoic 

 era. The rise and fall of the trilobites is shown graphically in the 

 curve of the accompanying figure (Fig. 156). Their climax appears 

 to have been reached by a rapid evolutionary ascent, and to have 

 been followed by a more gradual decline, but the ascent was doubt- 

 less very much more prolonged in reality than appears from the record, 

 which is doubtless less complete in the Cambrian and pre-Cambrian 

 systems than in those of later periods. The attainment of a climax 

 so early in the recorded history of life is a very notable event. 



The cystoids and the graptolites also reach their climaxes in this 

 period. The cystoids were, however, replaced by their near kin, the 

 crinoids, and the much simpler organization of the graptolites makes 



1 Walcott, Bull. Geol. Soc. of Am., Vol. Ill, pp. 153-171. 



