^ 



EXTERNAL FEATURES 



291 



structure, in the genus Ptei-ygotus (Fig. 164, 5) they agree closely 



with the second, third, and fourth pairs, 



and in Slimonia (Fig. 165, 5) they are 



nearly the same as the third and fourtli 



pairs. The sixth pair of appendages are 



much larger and more powerful than the. 



fifth pair in nearly all genera ; in Stylon- 



urus (Fig. 166), however, the sixth pair 



are similar to the fifth, both being greatly 



elongated and slender ; also in Eusarcus 



(Brepanoptencs) the sixth pair differ less 



from the preceding pair of appendages 



than is usually tlie case. 



In Pterygot'us there is a well-developed 

 epistome (Fig. 164, a) between the mouth 

 and the front margin of the carapace, 

 thus occupying the same position as tlie 

 hypostome of Trilobites (p. 233). The 

 metastoma is always well developed and 

 forms one of the distinguishing features 

 of the Eurypterids ; in form it varies from 

 oval in Eurypterus, to cordate in Slimonia, 

 and lyrate in Dolichopterus. 



The principal modifications seen in 

 the genital operculum are in the form 

 of its median process ; in Slimonia this 

 either ends in three sharp points posteriorly 

 (Fig. 165, c), or has the form of a trun- 

 cated cone ; its form in EiirypteriLS has 

 already been described. Glyptoseorpius 

 differs from other Eurypterids in the 

 possession of comb -like organs closely 

 resembling the pectines of Scorpions. 

 Slimonia apparently differs from other 

 genera in that the plate-like appendages 

 on the posterior three segments of the 

 mesosoma do not meet in the middle line 

 (Fig. 165, 10-12). In some forms, such 

 as Pterygotus (Fig. 164), there is a nearly 

 gradual decrease in the width of the 



Fig. 165. — SUmonia acu- 

 minata, Salter. Upper 

 Silurian. Restoration of 

 ventral surface, x ^,. 1- 

 6, Appendages of pro- 

 soma ; 7, 8, genital oper- 

 culum ; 7-12, mesosoma ; 

 13-18, segments of meta- 

 soma ; 19, tail-spine ; a, 

 lateral eye ; b, metastoma, 

 covering the inner parts 

 of the coxae of the last 

 pair of appendages ; c, 

 median pi-ocess of genital 

 operculum ; d, liranchial 

 lamellae seen through the 

 plate - like appendages. 

 (After Laurie.) 



segments in passing 



