2 92 ARACHNIDA EURYPTERIDA CHAP. 



from the mesosoma to the metasoma ; but in some others, 

 which in this respect are less primitive, such as Slimonia 

 (Fig. 165), the posterior five segments of the body (lilte those 

 of Scorpions) are distinctly narrower and longer than the preced- 

 ing segments. The long tail-spine of Eurypterus is represented 

 in Slimonia by an oval plate produced into a spine at the end 

 (Fig. 165,19); whilst in some species of Pterygotus the plate 

 is bilobed at the posterior end (Fig. 164, 19). In Hughmilleria 

 the tail-spine is lanceolate. 



The Eurypterids resemble the Xiphosura in many respects. In 

 both groups the prosoma consists of at least six fused segments, 

 and bears two pairs of eyes — one pair simple, the other grouped 

 eyes — on the dorsal surface of the carapace. The number and 

 position of the appendages of the prosoma in Eurypterids agree 

 with those of Limulus. The chelicerae are closely similar in both 

 cases. The coxae of all five pairs of legs in Eurypterids are 

 toothed and function in mastication ; similarly in Zimidus all are 

 spiny except the coxae of the last pair of legs. In both a similar 

 epicoxite is present on the coxae. The number of joints in the 

 legs is somewhat greater in the Eurypterids than in Limulus, 

 and in the former none of the legs end in chelae, whereas in the 

 latter all the walking legs, except the last, and also the first in 

 the male, may be chelate. The metastoma of Eurypterids differs 

 in being a large unpaired plate, but is represented in Limulus by 

 the pair of relatively small chilaria. On the mesosoma the 

 genital operculum and plate - like appendages with branchial 

 lamellae are similar in both groups, but in the Eurypterids 

 the genital operculum shows a greater development and covers 

 the second segment, which is without plate-like appendages. A 

 striking difference between the two groups is seen in the seg- 

 ments of the mesosoma and metasoma ; in Eurypterids these are 

 all free, whilst in Linndus they are fused together, but this 

 difference is bridged over by some of the Palaeozoic Xiphosura 

 (Fig. 159, A) in which those segments are free. 



The Eurypterids present a striking resemblance to Scorpions. 

 In both groups the segments in the three regions of the body are 

 the same in number, and the appendages of the prosoma also 

 agree in number and position. The pre -oral appendages are 

 chelate in both, but the second pair of appendages are chelate in 

 the Scorpions only. In Eurypterids the coxae of the five pairs 



