^^"^ THE NEW EVOLUTION '^^^'' 



now extinct — in fact the last of them disappeared 

 many millions of years ago. 



The remains of trilobites are extremely common in 

 the very oldest rocks in which are found fossil remains 

 of animals in a satisfactory state of preservation. 

 They are abundant in the rocks of the Cambrian 

 period, in which they exceed in number and in di- 

 versity the remains of all the other forms of animal 

 life. In the succeeding period, the Ordovician, they 

 were also very numerous and varied. They were less 

 numerous and varied in the Silurian, and during the 

 Devonian they declined in numbers and in variety. 

 In the Carboniferous only a few, all rather closely 

 related to each other, are found, and at the end of this 

 period they entirely disappeared. 



Over two thousand different kinds of trilobites are 

 known. These vary in length from less than half an 

 inch to nearly two feet. The trilobites represent the 

 only large subdivision of the jointed-legged animals 

 or Arthropoda which has become extinct. 



In the phylum Arthropoda there is another much 

 smaller group now wholly extinct which is of special 

 interest as it includes the largest members of the 

 phylum. This group is that containing the so-called 

 eurypterids (fig. 31, p. 55), some of which were nearly 

 ten feet long. The largest member of the phylum 

 Arthropoda at the present time is the Japanese giant 

 spider-crab which measures eleven feet or more from 

 claw to claw, but its body is relatively small. The 

 eurypterids are first known from the Cambrian, and 

 the last of them are found in the Carboniferous. 



