Eurypterida, an extinct order of Arachnida, which occur principally in the Upper Silurian and Devonian rocks. The three best-known genera are Eurypterus, Ptergotus, and Stimouia. They were all marine, and swam about by means of four pairs of large appendages around the mouth. There were no limbs on the abdomen, but this ended in a large, expanded telson. The order has certain affinities with the Trilobites, and both were once included among the Crustacea.