XI 



EXTERNAL FEATURES 



29! 



5 



structure, in the genus Pterygotus (Fig. 164, 5) they agree closely 



with the second, third, and fourth pairs, 



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



nearly the same as the third and fourth 



pairs. The sixth pair of appendages are 



much larger and more powerful than the 



fifth pair in nearly all genera ; in Xtt/lon- 



n r us (Fig. 166), however, the sixth pair 



are similar to the fifth, both being greatly 



elongated and slender ; also in Eusarcus 



(Drepanopterus) the sixth' pair differ less 



from the preceding pair of appendages 



than is usually the case. 



In Pterygotus 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 the 

 hypostome of Trilobites (p. 2o3). The 

 metastoma is always well developed and 

 forms one of the distinguishing features 

 of the Eurypterids ; in form it varies from 

 oval in Eni-i/i>lt i' as, to cordate in Slimonia, 

 and lyrate in ZJo/ir/iajifcrus. 



The principal modifications seen in 

 the genital operculum arc, in the form 



of its median process; in Wninni'm this 

 either ends in three sharp points posteriorly 

 c or has the form of a trun- 





'Fig. 165, 



cated cone; its form in Kuri/i>terus has 

 already been described. Glyptoscorpius 

 differs from other Eurypterids in the 

 possession of comb-like organs close] \ 

 resembling the pectim-s of Scorpions. 

 X/i inn/tin apparently differs from other 

 genera in that the plate-like appendages 

 "ii the posterior three segments of the 

 niesosonia do not meet in the middle line 

 (Fii!;. 165, 10-12). In some forms, such 

 as I'll ri/'jot'HS (Fig. 164), there is a nearly 

 gradual decrease in the width of the 



;. lt)5. ,<<ti aunt in iii'ii- 



minata, Salter. I'pper 

 Silurian. lle-tcu-ation nl' 

 ventral surface, ,',. 1 

 ;. Appendages \ pi" 



7. B, '_'! i it at "per 

 ; 7- !'_', iiie-"-"ina : 

 1 :; ] S, sej merits "I nn-ta- 



lateral e\ e ; ' 'ina, 



roM-rillJ llie inner p 



dt' the coxae "I tlie 



pair nl ap] 



nieilian p it.il 



i>|ieri'iil'iiu : d, ' 'i .iin-hial 



lami llae i u thi 



jilate - h 



(After Laurie. 



si --mi 'Ml a 



in passing 



