208 



ARTHROPODA. EURYPTERIDA. 



free and movable segments which taper towards the 

 posterior extremity. The first six free segments (1 6) 

 are regarded by some as the abdomen, by others as the 

 thorax, and the remaining seven (7 12, #) as either the 

 abdomen or post-abdomen ; the last segment is usually an 

 oval pointed plate, the telson (g). The cephalothoracic 



FIG. 56. Ventral surface of Pterygotus osiliensis, from the Upper Silurian, 

 Rootzikiill. a, epistome ; b, metastoma ; c, first pair of appendages ; 

 d, compound eyes ; /, operculum ; g, telson ; 1' 5', second to sixth 

 pairs of appendages; I. V., ventral plates of the abdomen ; 7 12, 

 segments of the post-abdomen. Reduced. (After Schmidt.) 



