382 ARTHROPOD A. 



dages 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," Behind the head come thirteen 

 free segments, counting the telson as one. The first two of 

 these, immediately behind the carapace, are covered below 

 by a thoracic plate or "operculum," which doubtless pro- 

 tected the reproductive organs. The other somites carry no 

 appendages, though it is certain that some of them bore 

 membranous branchiae. The "telson" or terminal segment 

 of the abdomen (fig. 239, t) is sometimes lanceolate or bi- 

 lobate, as in Pterygotus and Slimonia, or sometimes narrow 

 and sword-shaped, as in Eurypterus and Stylonurus. 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. 



There seems to be no doubt that the berry-like bodies 

 which are found in the Old Eed Sandstone of Scotland, and 

 which have been described under the name of Parka de- 

 cipiens, are truly the eggs of large Crustaceans belonging to 

 the Eurypterids. 



Besides such typical Eurypterids as Pterygotus, Eurypterus, 

 and Slimonia, we have to refer to this order a number of 

 forms which may be regarded as in many respects transi- 

 tional between the Eurypterida and the Xiplwsura. The 

 most important of these connecting links is Hemiaspis (fig. 

 240, A), of the Upper Silurian, in which the head-shield 

 has a general resemblance to that of Limuhis, though the 

 eyes have not been determined. The thorax consists of six 

 segments, which are not amalgamated, but agree with those 

 of the Eurypterids in being free, and which resemble those 

 of the Trilobites in being markedly trilobed. There are 

 three free segments in the abdomen, and these are followed 

 by a long spine-like telson. Closely allied to the above, 

 especially in the distinct division of the body into a cephalic, 



