210 ARTHROPODA. EURYFfERIDA. 



Pterygotus. Cephalothoracic shield rounded in front; 

 the compound eyes are marginal, at the anterior angles. 

 The telson is oval with a pointed extremity. The pre- 

 oral appendages are long and chelate. Metastoma oval. 

 The examples of this genus are usually of enormous size, 

 P. anglicus sometimes reaching a length of nearly six feet. 

 Lower Ludlow to Old Red Sandstone. 



Slimonia. Cephalothoracic shield quadrate ; the com- 

 pound eyes at the anterior angles. The telson is oval, 

 ending in a point. Metastoma heart-shaped. The pre- 

 oral appendages short and not chelate. Upper Ludlow 

 and Passage Beds. 



Distribution of the Eurypterida. 



The order ranges from the Ordovician to the Permian, 

 and is most abundant in the Upper Silurian and Old Red 

 Sandstone. The chief genera are Eurypterus, Pterygotus, 

 Slimonia, and Stylonurus. 



