MEROSTOM'ATA (Gr. meron, thigh; stoma, mouth), an order of crustaceans com prising two sub-orders, eurypterida (Gr. eurus, broad; pteron, wing) and xiphosura (Gr. ziphos, svvord; oura, tail), the latter including the only living representative, the king-crab, or horse-shoe crab. The first of these sub-orders is extinct, and their fossils are exclusively paleozoic, all the members being confined to the Silurian, Devonian, and carboniferous formations. The sub-order eurypterida is described by Henry Woodward as composed of " crustaceans with numerous free thoracico-abdominal segments, the first and second of which bear one or more broad lamellar appendages upon their ventral surface, the remaining segments being devoid of appendages; anterior rings united into a carapace bearing a pair of larval eyes near the center, and a pair of large, marginal, or sub-central eyes; the mouth furnished with a broad post-oral plate or metastoma, and five pairs of movable appendages, the posterior of which form great swimming-feet; the telson, or terminal segment, extremely variable in form; the integument character istically sculptured." Some of the members of this sub-order were of gigantic dimen sions, as pterygotus anglicus, measuring 6 ft. or more in length. The berry-like bodies. found in the old red sandstone of Scotland, and described under the name of parka deeipiens, are regarded as the eggs of larue crustaceans of the eurypterid group. The second sub-order, xiphosura, are characterized by Woodward as follows: Crustacea having the anterior segments welded together to form a broad, convex buckler, upon the dorsal surface of which are placed the compound eyes and ocelli; the former sub centrally, the latter in the center in front. The mouth is furnished with a small labrum, a.
rudimentary mekstoma, and six pairs of appendages. Posterior segments of the body are more or less free, and bearing upon their ventral surfaces a series of broad lamellar append ages; the telson, or terminal segment, ensiform." The only living members of this sub-order are the iimu/i„ commonly known as king-crabs, horse-shoe crabs. They inhabit the Indian. and Japanese seas, the Ai:allies, and the coasts of North America. The xiphosura com menced their existence in the upper Silurian format,ion, where they are represented by the neolimulus faleatus of Henry Woodward. In this genus the head-shield has a. resemblance to that of the king crab, and there are traces of a divisional line crossing the head, and apparently corresponding with the facial suture of the trilobites (q.v.). Compound eyes and ocelli seem to be present, and there are six free thoracic, and probably three free abdominal segments, of which only two have been preserved. No members of the sub-order have been found in the Devonian formation, but several ty-pes occur in the carboniferous, the most important member being pestwiehia rotundifolia of the coal measures of Europe, and the genus euproops of the North American coal measures, very similar to each other, the latter, however, having eyes situated on the anterior edge of the cephalic buckler. Limuloid crustaceans are also found in the permian and triassic formations, as well as in the upper Jurassic, the cretaceous, and tertiary. See INITERTEBRATA.