348 ARACHNIDA. EURYPTERIDA 



than an actual member of, the Eurypterida. The lung- 

 books of the Scorpions are represented by the leaf-like 

 gills of the Eurypterids, but the plate-like appendages 

 of the mesosoma are absent in the Scorpions. In both 

 groups the segments of the metasoma are free and without 

 appendages and at the posterior end is a tail-spine. The 

 differences between the Eurypterids and recent Scorpions 

 are to some extent bridged over by Palceophonus, sl Silurian 

 Scorpion (see p. 353). 



The Eurypterids agree in many respects with Limulus. 

 The principal points of difference are : — (1) only the first 

 pair of appendages are chelate in Eurypterids, whereas in 

 Limulus all the walking-legs except the last, and the first 

 in the male, may be chelate ; (2) the last pair of legs are 

 larger in Eurypterids than in Limulus and their basal 

 joints assist in mastication; (3) the large, single plate 

 forming the metastoma in Eurypterids is represented by 

 the pair of small chilaria of Limulus ; (4) the second seg- 

 ment of the mesosoma in Eurypterids is without append- 

 ages and is covered by the genital operculum; (5) in the 

 abdomen all the segments are free in Eurypterids but 

 fused in Limulus, and in the latter the metasoma is much 

 reduced — these differences in the abdomen, however, are 

 bridged over by the Palaeozoic Xiphosura. 



Eurypterus. Prosoma (cephalothorax) quadrate, the an- 

 terior angles rounded ; the compound eyes are a little in front of 

 the median lateral point on each side. The tail-spine is long, 

 narrow, and pointed. The pre-oral appendages are small and con- 

 sist of a basal joint and a chela ; the second appendage consists of 

 seven joints, the remaining four pairs of eight joints ; all these live 

 pairs of appendages are without chelae. The second, third and 

 fourth pairs are similar in structure and bear spines ; the fifth pair 

 are longer than the preceding and without spines ; and the sixth 

 pair are much longer and also larger, with a large quadrate basal 



