EURYPTERIDA. 



797 



The sixth pair of limbs are large and paddle-like. The five 

 anterior abdominal segments bear plate-like appendages, over- 



TIG. 517. Eurypterus remipes after Nieszkowski. a Dorsal view ; & ventral view ; H meta- 



stoma ; eyes ; St. caudal spine. 

 N.B. In 6 the chelicerae are omitted and one segment too many is represented in the anterior 



abdominal region. 



lapping one another from before backwards, and are so arranged 

 that the exposed part of the fifth lies beneath the sixth tergal 

 plate. The first, known as the operculum, is traversed by a 

 transverse suture, so that it has the appearance of representing 

 two pairs of appendages. The appendages were separated 

 from one another by a suture in the middle line, and oval patches 

 on the upper surfaces of the four posterior pairs are apparently 

 the remains of foliaceous branchiae. 



One set of individuals, larger than the others, and regarded 

 by Holm as females, bear a prominent complex median organ 



has been demonstrated by Schmidt and by Holm in the material from 

 the island of Oesel in the Baltic. The fossils are here contained in 

 calcareous beds of the Silurian, and by dissolving the matrix with acid 

 these authors have succeeded in obtaining the chitinous elements free. 

 When mounted in Canada balsam, the preparations show the details of 

 structure in great perfection. 



