542 



GENERAL CONSIDERATIONS 



taceans appeared during the Cambrian and Ordovician. Myriapods 

 appeared in the Devonian, scorpions in the Silurian, and spiders in the 

 Carboniferous. Eurypterids, which were scorpion-Hke but marine, 



Fig. 373. — A trilobite, restored, show-ing limbs and antennae. Dorsal surface. (From 



LeConte, "Elements of Geology.") 



hved throughout the Paleozoic. They have considerable resemblance to 

 the king crabs, which appeared in the Triassic. Insects appeared first 

 in the Carboniferous, and the largest insects that have ever lived were in 



Fig. 374. — An ostracoderm, restored. {Based upon Schuchert, "Historical Geology," after 



Koken.) 



the Pennsylvanian, a dragon fly from the coal deposits of Belgium measur- 

 ing 29 inches across the wings. 



Ostracoderms (Fig. 374), the first vertebrates, appeared in the Ordo- 

 vician, and only they and primitive sharks are known from the Silurian, 

 but ganoids and lungfishes existed in great variety in the Devonian, 



