CLASS II 



AEACHNIDA— EURYPTERIDA 



781 



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

 Tlie first and second Segments of the mesosoma are covered on the ventral surface 

 by tlie genital operculum, wliich consists of a pair of plates meeting in the middle 

 line and liaving a median lobe attached to them. Tlie latter, frora 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 hroadly expanded in the mesosomatic re 

 Prosoma suhquadrate to suhtriangidar in outline, with rounded front angles ; telson 

 spiniform. Compound ey es smooth, 

 not facetted, gener ally near the 

 middle of the cephalic shield ; no 

 epistoma; chelicerae not extending 

 heyond the frontal margin of the 

 carapace. Sixth paif of legs 

 adapted for either smmming or 

 crawling. Femade genital append- 

 age composed of several lohes. 



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 (Encyclop. Brit., 

 12th ed., article on Arachnida) 

 theyare placedin sejmrate families. 

 Concerning geneticrelations, 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 

 Strabops from the Cambrian. 

 This genus is one of the earliest 

 known Eurypterids, and is re- 

 garded by Clarke and Ruedemann 



as an actual progenitor of most ^''^^'^(^^opsthacheri Beecher. Fotosi limestone (Upper Cambrian) ; 

 CM • X- * -,. ^^- Fran(;ois County, Mo. Restoration of dorsal aspect. 2/„ 



bllurian iorms. Accordmg to (after Clarke and Ruedemann). 



the authors just named the Euryp- 

 terids studied by them pass through a so-called Strahops-stage during the course of 

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

 Eurypterids 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. 



