ANATOMY AND RELATIONS OF THE ETJRYPTERIL\E. 515 



in width from those on the third segment, which overlap each other in the middle line, 

 to those on the sixth, each of which only occupied one-third of the breadth of the 

 segment, and which were probably placed at the outer sides. The evidence against there 

 having been similar appendages on the seventh free segment is purely negative, and, 

 therefore, with forms like this, not very conclusive. Comparison, however, with the 

 recent forms (Limulus and Scorpio) which seem to be nearly related to these fossils, 

 makes it, a priori, probable that only six of the free segments bore appendages. The 

 cylindrical form and reduced width of the last five segments renders it highly improbable 

 that they bore appendages. 



In fig. 9 I have attempted a restoration of Slimonia from the ventral side, showing 

 the position and form of the various appendages. 



Pterygotus. 



The resemblances in most respects among the Eurypterids are so great that it will only 

 be necessary, in the treating of the succeeding forms, to mention the points in which 

 they differ from the normal type. 



The carapace in Pterygotus is semicircular, and the compound eyes are marginal, a 

 small pair of ocelli being also present near the middle of the carapace. The body is less 

 differentiated into two regions than in Slimonia or Eurypterus, the abdomen passing into 

 the tail with very little constriction. 



The sclerites call for no special description, being simple and band-like, but the telson, 

 occurring, as it does, in various forms, demands a few words. The type most like that of 

 Slimonia, and probably the more primitive, is that found in Pt. anglicus and others. 

 In these the telson is somewhat spatulate, ending in a short spine. This form of telson is 

 usually strengthened by a longitudinal ridge down the middle line. The other extreme 

 in form is found in Pt. bilobus, &c, in which the telson is oval in form, and deeply 

 cleft at the posterior end. Some curious forms have been described from the Waterlime 

 Group in America. Pt. globicaudatus * has a simple round telson, while Pt. quadrati- 

 caudatus has, as its name implies, a more or less square one, slightly cleft in the 

 middle of its almost straight posterior margin. Personally I do not feel quite sure that 

 these peculiar forms may not be due to fracture or folding, and this especially with Pt. 

 globicaudatus. The forms with bilobed telson — which might be fairly separated from the 

 rest as a sub-genus, were it not that the frequent absence of the tail would make such an 

 arrangement highly inconvenient — are entirely confined to the Upper Silurian, and 

 include almost all the forms from that horizon, the acute-tailed ones being, with the 

 exception of Pt. acuticaudatusf and Pt. Cummingsii,\ which may prove to be one 

 species, confined to the Old Red and Devonian. 



* Pohlmann, Bull. Buff. Soc. Nat. Sci., vol. iv. t Pohlmann, loc. cit. 



+ Grote, Bull. Buff. Soc. Nat. Sci., vol. iii. 



