i>22 MR MALCOLM LAURIE ON THE 



The genealogical tree which I would suggest for this group is, then, as follows 



Slimoiiia. Eurypterus. 



Drepanoptenis. 



Stylonurus, 



? Trilobite. 



Slimonia has differentiated itself from Pterygotus chiefly by the greater development 

 of its genital organ and by the specialisation of the second pair of appendages for a 

 tactile function. Along with this it has also acquired, or, more probably, retained, the 

 short chelicerse, masticatory rather than prehensile in function, which are characteristic of 

 the other forms. 



Eurypterus specialises in the position of the eyes, which is common to all the forms 

 above a, and further, in the form of the tail segments and telson and in the specialisation of 

 the fifth appendage, which, however, is comparatively slight. 



Stylonurus develops from Eurypterus via forms probably most nearly represented 

 by Drepanopterus, by the greater specialisation of the fifth appendage, and the reduction 

 of the sixth appendage from the typical digging foot to a purely crawling one. This may 

 indicate more purely littoral habits, or a more active predatory existence, demanding 

 rapid locomotion rather than firm anchorage. 



Relations of Eurypterida to other Groups. 



In attempting to arrive at some conclusion as to the place in classification of the 

 Eurypterids, the Geological Record again gives us but little help. The contemporaneous 

 Arthropoda are for the most part very obscure, and in many cases appear to belong to 

 well-defined types. Some of these, such as the Scorpions and Pedipalpi, have persisted 

 almost unchanged till the present day ; while those which have died out, such as the 

 Trilobites and Anthracomarti, afford little information of morphological value, owing 

 chiefly to their imperfect state of preservation. I have not thought it necessary in the 

 following speculations to recapitulate at length the arguments adduced for and against the 

 relationship of Limulus to the Arachnida and Eurypterida, as these are well known and 

 easily accessible.* 



* Lankester ; Packard ; Woodward, &c. 



