ARTHROPODA. EURYPTERIDA. 



207 



large size, some having a length of several feet ; trilobation 

 is either absent or only very faintly shown. The body is 

 protected by a chitinous skeleton (fig. 55), the anterior 

 part (cephalothorax) consisting of a large quadrate or 

 semicircular shield ; this is followed posteriorly by thirteen 



FIG. 55. Dorsal surface of Pterygotus osiliensis, from the Upper Silurian, 

 Rootzikull. c, first pair of appendages ; d, compound eyes ; e, ocelli ; 

 g, telson; 5', fifth pair of appendages; 16, segments of the 

 abdomen; 712, segments of the post-abdomen. Reduced. (After 

 Schmidt.) 



