EURYPTERIDA. 



383 



a thoracic, and an abdominal region, the presence of free 

 thoracic segments, and the possession of a long tail-spine, 

 are the genera Pseudoniscus and JExapinurus of the Upper 

 Silurian (fig. 240, B and c.) 



The Eurypterids range from the Upper Silurian, where 

 they attain their maximum, through the Devonian, into 

 the Carboniferous rocks, where they appear to die out. 

 Traces, however, of these large Crustaceans are by no means 

 wanting in the Lower Silurian, though not as yet fully 



Fig. 240. A, Hemiaspis limuloides (after H. Woodward) ; B, Pseiudoniscits aculeatiis (after 

 Nieszkowski) ; c, Exo.pinurm Schrenkii (after Nieszkowski). All from the Upper Silurian. 



described. Of the typical genera, Plerygotus extends from 

 the Upper Silurian to the Upper Devonian, and species of 

 this genus seem to have attained a gigantic size. (Pterygotus 

 Anglicus is calculated to have reached a length of six feet.) 

 Slimonia is Upper Silurian, and Stylonurus is both Upper 

 Silurian and Devonian. Enrypterus is not known in the 

 Silurian, but is represented by many species in the De- 

 vonian, and extends into the Carboniferous rocks. Hemi~ 



