EURYPTERIDA BY HENRY WOODS, M.A. St. John’s College, Cambridge, University Lecturer in Palaeozoology. CHAPTER XI ARACHNIDA (_CONTINUED_)—DELOBRANCHIATA = MEROSTOMATA (_CONTINUED_)— EURYPTERIDA =Order II. Eurypterida.= The Eurypterida or Gigantostraca are found only in the Palaeozoic formations. Some species of _Pterygotus_, _Slimonia_, and _Stylonurus_ have a length of from five to six feet, and are not only the largest Invertebrates which have been found fossil but do not seem to be surpassed in size at the present day except by some of the Dibranchiate Cephalopods. All the Eurypterids were aquatic, and, with the possible exception of forms found in the Coal Measures, all were marine. The earliest examples occur in the Cambrian deposits, and the latest in the Permian; but although the Eurypterids have thus a considerable geological range, yet it is mainly in the Silurian and the Old Red Sandstone that they are found, the principal genera represented in those deposits being _Eurypterus_, _Stylonurus_, _Slimonia_, _Pterygotus_, _Hughmilleria_, _Dolichopterus_, and _Eusarcus_. From the Cambrian rocks the only form recorded is _Strabops_;[227] in the Ordovician the imperfectly known _Echinognathus_[228] and some indeterminable fragments have alone been found. In the Carboniferous deposits _Eurypterus_ and _Glyptoscorpius_ occur, and the former survived into the Permian.[229] [Illustration: FIG. 161.—_Eurypterus fischeri_, Eichw. Upper Silurian, Rootziküll, Oesel. Dorsal surface. _a_, Ocellus; _b_, lateral eye; 2–6, appendages of prosoma; 7–12, segments of mesosoma; 13–18, segments of metasoma; 19, tail-spine. (After Holm.) ] The Eurypterid which is best known is _Eurypterus fischeri_ (Figs. 161, 162), which is found in the Upper Silurian rocks at Rootziküll in the Island of Oesel (Gulf of Riga). In the Eurypterids from other deposits the chitinous exoskeleton has been altered into a carbonaceous substance, but in the specimens from Oesel the chitin is perfectly preserved in its original condition; and since these specimens are found in a dolomitic rock which is soluble in acid, it has been possible to separate the fossil completely from the rock in which it is embedded, with the result that the structure can be studied more easily and more thoroughly than in the case of specimens from other localities. Consequently _Eurypterus fischeri_[230] may, with advantage, be taken as a type of the Eurypterida.