Aglaspis, found in the Upper Cambrian of Wisconsin, has been regarded as a Xiphosuran. If that view of its position is correct, then Aglaspis will be the earliest representative of the group at present known. Other genera of Palaeozoic Xiphosura are Bunodes, Bunodella, and Pseudoniscus in the Silurian; Protolimulus in the Upper Devonian; and Prolimulus in the Permian.
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]]
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.)