MER'OSTOM'ATA (Neo-Lat. nom. pl., from Gk. uepos, meros! part + ar6i4a, stoma, mouth). A class of Arthropoda, standing next above the Trilobites and immediately below the Arachnida, these three classes forming a series by them ! selves and distinct from the Crustacea. They are represented by the king-crab (q.v.), or LimuIns, the sole surviving member of the class. The merostomes are subdivided into three orders: the Eurypterida, represented by Eurypterus; the , Synziphosura, of which three Paleozoic families arc the types; and, third, the Xiphosura, type Limulus. The class chiefly differs from Trilo bites in having appendages of two types, those of the head being single, those of the abdomen being biramous; in being provided with book rills, attached to the broad abdominal legs, which are fused together at the base, the head appen dages often ending in a forceps, while they dif fer from the Arachnida in breathing by gills, I all the forms being marine, in the nature of the appendages, the brain, the nervous cord envel oped by arteries, and by the reproductive organs. The earliest forms are the Eurypterida. The typical genus Eutypterus, unlike the king crab, probably actively swarm nearer the sur face in of the sea. The species are found fossiln rocks of Cambrian to Permian age. The form of the body is somewhat like that of a scorpion, though flatter and of larger size. A quadrate headpiece or cephalothorax with rounded front corners bears two large reniform compound eyes, between which are two small eye spots or (men The abdominal portion consists of twelve seg ments that taper posteriorly and are terminated by a strong, sharp spine or nelson. The structure of the ventral surface of the body is quite similar to that of the horse-shoe crab.
The eurypterids appeared first in the Potosi Cambrian limestones of Missouri. At the end of the Silurian period geographic conditions seem to have favored their development. for they expanded rapidly and became the dominant types of the fauna of the inclosed basins in which were deposited the shallow water passage beds between the Silurian and Devonian forma tions. They appear in great numbers in the water limestones or cement rocks of New York State, and in beds of equivalent age and similar character in Great Britain and the Baltic Provinces; also in the coal meas ures of Carboniferous age in Pennsylvania, Nova Scotia, and in Europe, where they are associated with the fossil remains of a swamp fauna and flora. The last member of the genus is known from Permian fresh-water The genus seems to have been first a marine shallow-water organism and to have changed its habitat through brackish and possibly to fresh water in succeeding geological periods. Several allied genera are found asso ciated with the remains of Eurypterus; of these Pterygotus, Slimonia, and Stylonurus are the most important. See the articles KING-CRAB; XIPHOSURA.
BIBLIOGRAPHY. Zittell, Text-Book of PaleonBibliography. Zittell, Text-Book of Paleon- tology, trans. by Eastman (New York, 1900) ; Woodward,Ilionogroph of the British. Fossil Crus tacca of the Order Merostomuta- (Palxontological Society, London, 1S66-7S); Packard, "On the Carboniferous Xiphosurous Fauna of North America," in the Memoirs of the .Vational Acad emy of Sciences, vol. iii. (Washington. 1SS6) ; with the writings of De Kay, Ball. Huxley, Salt er, Peach, and Laurie.