SEGMENTATION 



289 



and of a pair of long pointed pieces which project on to the next 

 segment. Just as in the ease of the genital operculum the basal 

 part is united iu front to the appendages, the remainder being 

 free, and separating the greater part of the two plate-like append- 

 ages. In the complete animal the median process of this segment 

 is covered by the median process of the genital operculum. The 

 remaining appendages of the female, and all the appendages 

 behind the operculum in the male, are without any median 



B -r^ 





Fig 163 —linryptmis fiscUn, Eiehw. Upper Silurian. (After Holm. ) A, Genital 

 operculum of male ; «, median process. B, IVIiddle part of second appendage of 

 the mesosoma in tlie female, showing the median process. 



process, and the plates of each pair unite by a suture in the 

 middle line. 



The metasoma (Fig. 161, 13-18) consists of six segments 

 which become longer and narrower from before backwards. Each 

 segment is covered by a ring-like sheath or sclerite, and bears no 

 appendages. The posterior end of the last segment is produced 

 into a lobe on each side, and between these lobes the long, narrow 

 tail-spine arises (Fig. 161, 19). 



The other genera of the Eurypterida do not differ in any 

 important ■ morphological respects from the form just described, 

 VOL. IV ^ 



