The Eurypterids present a striking resemblance to Scorpions. In both groups the segments in the three regions of the body are the same in number, and the appendages of the prosoma also agree in number and position. The pre-oral appendages are chelate in both, but the second pair of appendages are chelate in the Scorpions only. In Eurypterids the coxae of the five pairs of legs are toothed and meet in the middle line, but in the Scorpions the coxae of the last two pairs do not meet; this difference, however, appears to be bridged over in the earliest known Scorpion—_Palaeophonus_,[236] from the Silurian rocks. The Eurypterids are distinguished from the Scorpions by the much greater development of the last pair of legs. The large metastoma of the former is homologous with the sternum of the Scorpion. The genital operculum is much smaller in Scorpions than in Eurypterids, and in this respect the latter agree with _Thelyphonus_ (one of the Pedipalpi) more than with the Scorpions. The pectines are absent in the Eurypterids except in _Glyptoscorpius_. Instead of the lung-books of the Scorpions the Eurypterids possess branchial lamellae on the plate-like appendages; but this difference between the two groups appears to be bridged over by _Palaeophonus_, which was marine, and may have possessed branchial lamellae since stigmata seem to be absent. _Glyptoscorpius_,[237] which is found in the Lower Carboniferous of the south of Scotland, is a form of considerable interest. It is about a foot in length, and agrees in many respects with Eurypterida, but it may be necessary to separate it from that group since it possesses pectines, and the legs end in a double claw; it cannot, however, be regarded as a link between Eurypterids and Scorpions, but must rather be considered as an offshoot from the former, since the latter group was already in existence at a much earlier period. ARACHNIDA EMBOLOBRANCHIATA (SCORPIONS, SPIDERS, MITES, ETC.) BY CECIL WARBURTON, M.A. Christ’s College, Cambridge; Zoologist to the Royal Agricultural Society CHAPTER XII ARACHNIDA (_CONTINUED_)—EMBOLOBRANCHIATA—SCORPIONIDEA—PEDIPALPI SUB-CLASS II.—EMBOLOBRANCHIATA.[238] =Order I. Scorpionidea.= _Segmented Arachnids with chelate chelicerae and pedipalpi. The abdomen, which is broadly attached to the cephalothorax or prosoma, is divided into two regions, a six-jointed mesosoma and a six-jointed tail-like metasoma, ending in a poison-sting. There are four pairs of lung-books, and the second mesosomatic segment bears a pair of comb-like organs, the pectines._