CLASS II 



ARACHNIDA— EURYPTERIDA 



779 



of nearly three metres. The presence of gills upon the branchial appendages shows 

 that the Eurypterids were aquatic, and the structure of their appendages indicates 

 tliat they were for the most part mud-crawlers, though some were good swimmers. 

 They are found associated 

 with cephalopods and tri- 

 lobites in the Cambrian 

 and Ordovician of North 

 America ; with cephalo- 

 pods and marine arthropods 

 (Phyllocarids and Ostra- 

 cods) in the Silurian ; with 

 Ostracophores and Arthro- 

 dires in the Devonian ; and 

 with land plants, scorpions, 

 insects, fishes, and fresh- 

 water amphibians in the 

 productive Coal Measures. 

 It is ajiparent, therefore, 

 that from being originally 

 marine forms, they became 

 gradually adapted to 

 brackish, and jiossibly even 

 fresh-water conditions. 



The Eurypterids and 

 Xiphosures present a num- 

 l)er of points of common 

 resemblance. Both groups 

 have a prosoma comi^osed 

 of at least six fused seg- 

 ments, and bearing two 

 pairs of eyes, one pair 

 simple, the other com- 

 pound, on the dorsal sur- 

 face of the carapace. The 

 number and. position of 

 the appendages of the 

 prosoma in Eurypterids 

 are the same as in Limulus, 

 and the chelicerae are 

 similarly constructed in 



both cases. The basal joints of all five pairs of legs in Eurypterids are toothed and 

 function in mastication ; similarly in Limulus all are spiny except the coxae of the 

 last j)air of legs. In both groups a similar process called the epicoxite is borne upon 

 the coxae. On the mesosoma the genital operculum and plate-like appendages with 

 branchial lamellae are similar in both groups. A striking difference between them, 

 however, is seen in the segments of the mesosoma and metasoma, the somites being all 

 free in Eurypterids, but in Limulus fused together. The resemblance between Euryp- 

 terids and Scorpions is none the less striking, both groups showing the same number 



Fig. liiOo. 



Eurypterus remipes Dekay. Bertie Waterliine (Silurian) ; Herlvimer 

 County, N.Y. Restoration of dorsal aspect. Va (after Clarice and 

 Ruedemann). 



State Palaeont., 1903. — Schmidt, F., tJber Stylouurus von Osel. Bull. Acad. Imp. Sci. St-Petersb., 

 1904, ser. 5, vol. xx. — Seemann, F., Zur Gigantostrakenlauna BcJlimens. Beitr. Pal. u. Geol. 

 Osterr.-Uug., 1906, vol. xix. — Pruvost, P., Crustaces du nord de la France. Ann. Soc. Geol. 

 Nord, 1911, vol. xl. — Clarke, J. M., and Ruedemann, R., Eurypterida of New York. Mem. 

 N.Y. State Mus. no. xiv., 1912. 



