CRUSTACEA. 173 



appendages, the posterior of which form great swimming-feet ; the 

 telson, or terminal segment, extremely "variable inform; the in- 

 tegument characteristically sculptured" (Henry Woodward.) 



In the typical Eurypterids, such as Pterygotus (fig. 118) and 

 Eurypterus, the anterior portion of the body is covered by a 

 buckler or carapace, which bears a pair of minute larval eyes, 

 and a pair of large compound eyes placed marginally or sub- 

 centrally. On the under surface of the carapace are five pairs 

 of appendages. The first pair of these is usually regarded as 

 representing the antennae. The appendages of this pair are 

 mostly chelate, or converted into nipping-claws, but they are 

 sometimes simple, and they sometimes are spinous towards 

 their bases, and officiate as masticatory organs (Eurypterus 

 and Slimonid). The next three pairs of appendages are simply 

 pointed spinous organs (" pedipalps "), but the last pair is 

 sometimes converted into rowing-organs (Stylonurus}. The 

 last pair of appendages constitute two greatly-developed 

 swimming-feet, the bases of which are furnished with spines, 

 and form powerful jaws. The bases of these jaw-feet are covered 

 by a greatly-developed post-oral plate or " metastoma." Be- 

 hind the head come thirteen free segments, counting the telson 

 as one. The first two of these, immediately behind the cara- 

 pace, are covered below by a thoracic plate or " operculum," 

 which doubtless protected the reproductive organs. The 

 other somites carry no appendages, though it is probable that 

 some of them bore membranous branchiae. The " telson " or 

 terminal segment of the abdomen (fig. 118, t\ is sometimes 

 lanceolate or bilobate, as in Pterygotus and Slimoiiia, or some- 

 times narrow and sword-shaped, as in Eurypterus and Stylon- 

 urus. The surface of the crust is sculptured over the greater 

 part of its extent, with characteristic markings, which look 

 something like the scales of an ordinary Bony fish. These 

 " scale-marks," however, are often wanting over parts of the 

 surface. 



The Eurypterids range from the Upper Silurian, where they 

 attain their maximum, through the Devonian, into the Car- 

 boniferous Rocks, where they appear to die out. Traces, how- 

 ever, of these large Crustaceans are by no means wanting in 

 the Lower Silurian, though as yet undescribed. Of the typical 

 genera, Pterygotus 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. Euryp- 

 terus is not known in the Silurian, but is represented by many 



