ARACHNIDA. EURYPTERIDA 347 



The six segments of the mesosoma bear on the ventral 

 surface five pairs of plate-like appendages (fig. 148, 1. — v.; 

 fig. 149, VII. — xii.), each of which overlaps the one behind 

 like the tiles on a roof, and on the posterior (or inner) surface 

 of which are the leaf-like gills (fig. 149, c). The first pair 

 of plates form the genital operculum, and are divided 

 in the middle by a median process, which often extends 

 beyond the posterior margin of the operculum on to the 

 next pair of appendages ; the shape and size of the 

 median process differ in the two sexes. The genital 

 operculum covers the ventral surfaces of both the first 

 and second segments of the mesosoma (fig. 149, vn., viii.). 

 The segments of the metasoma (figs. 148, 7 — 12; 149, 

 xiv. — xviii.) are protected by continuous chitinous rings 

 and bear no appendages. 



In many respects the Eurypterids resemble the 

 Scorpions. The number of segments in each of the three 

 regions of the body is the same, and the two pairs of eyes 

 are similar in character and position. In both Eury- 

 pterids and Scorpions the prosoma bears six pairs of 

 appendages, of which the first are pre-oral and chelate, 

 and the remaining five agree in position and in general 

 form ; but in the Eurypterids the number of joints in the 

 walking legs varies, and the basal segments of all serve as 

 jaws, whereas in the Scorpions the last two pairs function 

 only in locomotion; also in the Eurypterids the last leg 

 and the genital operculum are much larger relatively than 

 in the Scorpions. One of the characteristic features of 

 the Eurypterids is the large metastoma; this is repre- 

 sented by the small sternum of the Scorpions. The 

 pectines are absent in the Eurypterids, except in Glypto- 

 scorpius from the Carboniferous — and this form should 

 probably be regarded as a specialised offshoot from, rather 



