120 BRITISH FOSSIL CRUSTACEA. 



The series of forms, already figured and described, and included in the genera 

 Pterygotus and Slimonia, although differing among themselves in many important points 

 of structure, have one common distinctive character, namely, the position of the large 

 compound eyes. 



In the genera Stylonurus and Eurypterus, and in the Xiphosura (to be described in 

 Part IV), these organs occupy a more or less sub-central position upon the dorsal surface 

 of the carapace, as is the case also in the Trilobita. In Pterygotus and Slimonia, on the 

 contrary, the eyes are marginal, being placed upon the latero-anterior border of the head. 

 The only form which, from the position of the eyes, can be compared with Slimonia and 

 Pterygotus, is Hall's Bolichopterus} an American form, in which the large compound eyes 

 approach very near the latero-anterior angles of the carapace ; but, nevertheless, they are 

 not marginal eyes, but are really placed upon the dorsal surface of the head, as in 

 Eurypterus proper, of which, indeed, Hall makes his Bolichopterus macrocheirus a sub- 

 generic form. 



For the greater convenience of the palaeontological student, the diagnostic characters of 

 each genus of the Merostomata will be given in a condensed form at the conclusion of 

 Part IV. 



1 'Geological Survey of New York, Palaeontology,' by James Hall, vol. iii, p. 414*, pi. lxxxiii, 

 Albany, U.S., 1859. 



