CLASS II 



ARACHNIDA— EUEYPTERIDA 



779 



of nearly tliree metres. The presence of gills lipon the brancliial appendages shows 



that tlie Eurypterids were aqiiatic, and the structure of their appendages indicates 



that they were for the most pait luud-crawlers, though sonie were good swinimers. 



They are found associated 



with cephalopods and tri- 



lobites in the Canibrian 



and Ordovician of North 



America ; with cejjhalo- 



pods and marine arthropods 



(Phyllocarids and Ostra- 



cods) in the Silurian ; with 



Ostra(3ophores and Arthro- 



dires in the Devonian ; and 



with land plants,scorpions, 



insects, fishes, and fresh- 



water amphibians in the 



productive Goal Measures. 



It is apparent, therefore, 



that from being originally 



marine forms, they became 



gradually adapted to 



brackish, and possibly even 



fresh-water conditions. 



The Eurypterids and 

 Xiphosures present a num- 

 ber of points of common 

 resemblance. Both groups 

 have a prosoma composed 

 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 pair of legs. In both groups a similar process called the epicoxite is borne upon 

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

 brancliial 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 



Earypterus 

 County, N.Y. 

 Ruedemann). 



Fig. 1505. 



'emipes Dekay. Bertie Waterlime (Silurian) ; Herkimer 

 Restoration of dorsal aspect. 1/2 (after Clarke and 



State Palaeont., 1903. — Schmidt, F., Über Stylouurus von Ösel. Bull. Acad. Imp. Sei. St-Petersb., 

 1904, ser. 5, vol. xx. — Seemann, F., Zur Gigantostrakenfauna Böhmens. Beitr. Pal. u. Geol. 

 Österr.-Ung., 1906, vol. x'w. — I'iurost, 1\, Crustaees du nord de la France. Ann. See. Geol. 

 Nord, 1911, vol. xl. — Clarhr, J. i/., and Ruedemann, R., Eurypterida of New York. Mem. 

 N.Y. State Mus. 110. xiv., 1912. 



