436 



HISTORICAL GEOLOGY 



period the class attained its greatest development (Fig. 408 A-I), 

 more than half of all the known genera of trilobites being represented 

 at that time. During the remainder of the Paleozoic they gradually 

 declined until their extinction was reached in the closing stages. 

 When Cambrian and Ordovician trilobites are compared, it is seen 

 that the latter have rounder eyes, that the tail shield (pygidium) 

 is larger, and that they have acquired the ability to roll themselves 

 up (Fig. 408 F, H y I) and thus protect the lower portions of the body, 

 many being found in this position which was apparently taken at the 

 approach of death. The Ordovician trilobites were not as large as 

 some Cambrian species, the maximum length being about 18 inches 

 as compared with about two feet for one Cambrian species. 



Other Arthropods continued from the Cambrian. Ostracods (Fig. 

 408 /), small bivalve crustaceans, flourished during portions of the 

 period, and also eurypterids (p. 449). 



Fishes 



An important addition to the fauna of the Ordovician, and one 

 which had a profound effect on the evolution of life in subsequent 

 ages, was the fishes, remains of which have been 

 found in Colorado and Wyoming. 



Plants 



Seaweeds. — Our knowledge of the plants of 

 the Ordovician is almost as incomplete as that 

 of the Cambrian. No land plants have been 

 found and no marine plants higher than sea- 

 weeds (Fig. 409) and calcareous algae. The 

 absence of land plants in Ordovician strata, 

 however, does not prove that a land vegetation 

 was lacking, since the known plant-bearing strata 

 are all of marine origin and consequently the 

 Fig. 409. — Ordovi- absence of land plant fossils would not be re- 

 C phycJl^foliul) P ^^ markable, even though land plants had been 

 .n< probably append- abundant. 

 ages of floating algae. 



Summary 



Progress and Character of Ordovician Life. — The life of the period 

 as shown by the fossils was fuller, more varied, and of a higher grade 

 than that of the Cambrian. Trilobites, cephalopods, gastropods, 



