EURYPTERUS, , a remarkable fossil arthropod related to the horseshoe crab (Limulus)any genera and species of which P occur in Paleozoic rocks of western Europe and eastern North America. They include the largest anthropods known, and form the family Eurypteride and order Eurypterida of the sub class Merostomata (q.v.). They resembled the modern horseshoe crabs in structure, but had elongated, often scorpion-like bodies, terminat ing in a hinged, spike-like or flattened tail or telson. The most remarkable feature, however, is the great size they attained, some exceeding six feet long, so that they were well named by Haeckel Gigantostraca. Tice surface was formed by a thin chitinous epidermal skeleton ornamented by fine scale-like markings, and bearing upon the head-shield two large lateral faceted eyes and a pair of median ocelli. Be neath the cephalo-thorax are six pairs of legs, the foremost preoral, the basal joints of which serve as jaws. The last pair is greatly enlarged, somewhat flattened and terminated by an oval plate, which suggests that these limbs served as paddles in swimming, but they may have been otherwise useful. In Pterygotus and some allied
genera the preoral limbs are modified into more or less antenna-like organs terminating in toothed pincers (chelae), no doubt for seizing prey, etc. The ventral segments are 13, of which the first two bear the genital organs, and the remainder leaf-like structures regarded as respiratory and equivalent to the thookgills' of Limulus. These extraordinary crustaceans are found associated with graptolites, cephalopods and trilobites in the Ordovician; with marine crustacea in the Silurian; with oceanic fishes in the Devonian, and with land and fresh-water plants and animals in the coal measures. Their structure shows that they must have been marine and good swimmers; but toward the end of their race they became gradually adapted to brackish and even fresh water. The latest review of the group is in Eastman's American edition of Zittell's 'Text-book of Palzon tology' (1900).