100 



HISTORICAL GEOLOGY 



Arthropods. — Crustaceans were represented by both Trilobites 

 and Eucrustaceans. Trilobites, which were the chief Ordovician 

 Arthropods, reached their climax or culmination of development 

 in numbers and species, more than a thousand species being 

 known from the Ordovician alone (Fig. 58). These animals, after 

 the Brachiopods, appear to have been among the most numerous 

 animals of the time. Their variation in size was much like that of 

 the Cambrian, but their eyes were usually larger and better 

 developed. Eucrustaceans were represented by comparatively 

 few simple forms, e.g. Ostracods and Cirripeds (Barnacles). 



a b 



Fig. 57 

 Bits of Ordovician sea-bottom: a, Brachiopod shells on limestone; b, Crinoid, 

 Bryozoan, Brachiopod, Pelecypod, Gastropod, and Cephalopod remains in 

 calcareous sandstone. (W. J. Miller, photos.) 



Arachnids, which date from Algonkian time, were represented, 

 though not abundantly, by the remarkable group of Eurypterids. 

 Since these creatures reached a much fuller development during the 

 Silurian period, further discussion is reserved for the next chapter. 



Insects are not known to have existed during this period. 



