ARACHNIDA. EURYPTERIDA 



349 



joint. The metastoma is oval. 

 The median process of the genital 

 operculum is short in the male, 

 long in the female. Lower Lud- 

 low to Permian. Ex. E. fisckeri, 

 Upper Silurian. 



Stylonurus. General form 

 similar to Pterygotus. Second, 

 third, and fourth pairs of append- 

 ages with spines ; the two poste- 

 rior pairs very long and slender. 

 Compound eyes near the middle 

 of the prosoma. Tail-spine long, 

 pointed. Body sometimes nearly 



5 feet long. Upper Silurian and 

 Old Red Sandstone. Ex. S. pow- 

 riei, same range. 



Pterygotus (figs. 147, 148). 

 Prosoma rounded in front ; the 

 compound eyes are at the mar- 

 gins. The tail-plate is oval and 

 either bilobed or pointed at its 

 extremity. The pre-oral append- 

 ages are long and chelate ; the 

 second, third, fourth and fifth pairs 

 are similar to each other in size 

 and structure ; the sixth pair long 

 and stout. Metastoma oval. The 

 examples of this genus are often 

 of enormous size, P. anglicus 

 sometimes reaching a length of 



6 feet. Lower Ludlow to Old 

 Red Sandstone. Ex. P. anglicus. 

 Old Red Sandstone ; P. bilobus, 

 Upper Silurian. 



Slimonia (fig. 149). Pro- 

 soma quadrate ; the compound 

 eyes at the anterior angles. Seg- 



Fig. 149. Slimonia. Restoration 

 of the under surface by M. 

 Laurie, b, metastoma ; c, leaf- 

 like gills seen through the ven- 

 tral plate-like appendages of 

 the mesosoma; g, tail -plate ; 

 I. — VI., appendages of the pro- 

 soma ; VII. — XII., segments of 

 the mesosoma; XIII. — XVIII., 

 segments of the metasoma ; 

 VII. — VIII., genital operculum. 

 Reduced. 



