THE SILURIAN PERIOD 



449 



Fig. 421. — Table showing the history of the trilobites. It is seen that the class 

 did not gradually increase and then gradually decrease, but that there were times 

 during which the species and individuals greatly increased and others in which, for a 

 time, there was a decrease. 



of the class did not differ greatly from that of the preceding period. 

 The most significant change from the Ordovician was in the dis- 

 appearance of Ordovician genera. 



Eurypterids. — The arthropods (Greek, arthron, joint, and pons, 

 foot), the branch of which the crustaceans and insects are members, 

 reached their greatest size in the eurypterids (Fig. 422). Some of the 

 Silurian forms at- 

 tained a length of one 

 and a half feet, while 

 in the Devonian there 

 were giants eight feet 

 long. They, together 

 with the giant cephal- 

 opods, were prob- 

 ably the terrors of 

 the sea until the fish 

 obtained the mas- 

 tery. They had elon- 

 gated bodies covered 

 with a leathery or 

 horny integument. 

 On the under side 

 were six pairs of legs, 

 of which the first had 

 large or small pincers. 

 Eurypterids are re- 

 lated to the horse- 

 shoe crabs (Limulus). 

 The presence of gills and their association with cephalopods and 

 trilobites in the Ordovician show that they lived in water and were 

 for the most part mud crawlers, although some were good swimmers. 



Fig. 422. — Silurian eurypterid : Dolichopterus macro- 

 cheirus. (After J. M. Clarke.) 



