CLASS II 



AEACHNIDA— EURYPTEKIDA 



781 



1509), and in a general way are comijarable to the leaf -like external gills of Limulus. 

 The first and second segments of the mesosoma are covered on the ventral snrface 

 by the genital operculum., which consists of a pair of plates meeting in the middle 

 line and having a median lobe attached to them. The latter, from analogy with 

 Limulus, is undoubtedly genital in function, and varies in form in the same species, 

 correlating with sex. 



Family 1. Eurypteridae Burmeister. 



Body elongate, narrow in form to hroadlij expanded in the mesosomatic region. 

 Frosoma subquadrate to suhtriangvlar in outline, with rounded front angles ; telson 

 spiniform. Compound eyes smooth, 

 not facetted, generally near the 

 middle of the cephalic shield ; no 

 epistoma; chelicerae not extending 

 beyond the fronted margin of the 

 carapace. Sixth pair of legs 

 adapted for either sioimming or 

 crawling. Female genital append- 

 age composed of several lobes. 



Certain genera which are here 

 included in this family {Eusarcus, 

 Stylonurus, etc.) present rather 

 wide departures from the type, and 

 in the recent classification proposed 

 by Lankester {Encydop. Brit., 

 12th ed., article on Arachnida) 

 they are placed in sejiarate families. 

 Concerning genetic relations, much 

 new light has been gained through 

 study of the stages of development 

 of the principal genera, and by 

 comparison of them with the 

 primitive and much generalised 

 Strahops from the Cambrian. 

 This genus is one of the earliest 

 known Eurypterids, and is re- 

 garded by Clarke and Euedemann 

 as an actual progenitor of most 

 Silurian forms. According to 

 the authors just named the Euryp- 

 terids studied by them pass through a so-called Straho2}s-sta.ge during the course of 

 their nepionic development. It has also been shown by them that the ontogeny of 

 Euiypterids fully corresponds to that . of Limulus in lacking any indication of a 

 nauplius or zoea stage. 



Strahops Beecher (Fig. 1507). Prosoma small, comparatively wider than 

 in Eurypterus, but the eyes further back, small, and very far apart ; body 

 somites not distinctly differentiated into two regions (mesosoma and 

 metasoma), twelve in number besides the short and blunt tail-spine. In view 

 of its generalised characters this genus is eminently fitted to serve as a 



Fig. 1507. 



Strabopfi thacheri Beecher. Potosi limestone (Upper Cambrian) ; 

 St. Franijois County, Mo. Restoration of dorsal aspect. 2/3 

 (after Clarke and Ruedemaun). 



