MEROSTOMATA. 555 



sentative of the order ( Campy locephalus) has been recorded from the 

 Permian rocks of Russia. With the close of the Palaeozoic period, 

 however, the order appears to have undergone complete extinction. 



The four most important genera of the Eurypterids are Eurypterus, 

 Pterygotus, Slimonia, and Styloniims. In Enrypterus (fig. 415) there 

 are five pairs of legs attached to the under surface of the head-shield, all 

 of which have their bases modified to act as masticatory organs, and the 

 last pair of which are developed into powerful swimming-paddles. There 

 is also, according to Fr. Schmidt, an additional, praeoral pair of appen- 

 dages, in the form of delicate jointed filaments, which represent the 

 antennae but are not chelate. Most of the species of Eurypterus are 

 found in the highest deposits of the Silurian rocks ; but there are a few 

 Devonian species, and the Carboniferous rocks have likewise yielded 

 representatives of the genus {e.g., the E. Scouleri of the Scotch Car- 

 boniferous rocks). 



The genus Pterygotus (fig. 416) is principally Silurian and Devonian, 

 the largest species {Pterygotus Anglicus) — sometimes attaining a length 

 of nearly six feet — being found in the Old Red Sandstone of Scotland ; 

 but Barrande has described forms of the genus from the Ordovician 

 rocks of Bohemia. Pterygotus differs from Eurypterus, among other 

 characters, in the marginal position of the large compound eyes, and 

 in the fact that the praeoral appendages (antennae) are very long and 

 terminate in nipping-claws. The telson of Pterygotus, further, is broad 

 and lanceolate, whereas that of Eurypterus is long and pointed. 

 The Silurian genus Slimo?iia resembles Pterygotus in the form of the 

 telson and in the marginal position of the compound eyes, but differs 

 from it in the quadrate form of the head-shield and in the structure of 

 the limbs. Finally, the Silurian and Devonian genus Stylonurus resem- 

 bles Eurypterus in the possession of an ensiform telson, and in its general 

 structure ; but the last two pairs of limbs are developed into long slender 

 rowing-organs, which reach nearly to the hinder extremity of the body. 



