SECT. XIV 



THE EURYPTERID/E 



539 



The head limbs are different in the two groups, 

 Pterygotus and Eurypterus (see Figs. 55 and 56). 

 They are, however, only different modifications in 



f 



Fig 5:5.— Pterygotus Osiliensis, upper Silurian, after F. Schmidt (from Zittell), show- 

 in- five pairs of cephalic limbs, the enormously developed first pair of antenna;, and 

 first trunk limbs in which the dorsal and ventral parapodiaare greatly developed 

 as locomotory and masticatory limbs respectively. The second pair of cephalic 

 limbs sometimes disappear, as in Pterygotus Anglicus Agassiz. 



adaptation to slight differences in the manner of 

 life. Judging from the forms of these remarkable 

 animals, we think the following method of explain- 



