any of an extinct order (Eurypterida) of large, aquatic arthropods of the Paleozoic Era, similar to the horseshoe crab and sometimes reaching a length of c. 1.8 m (c. 6 ft)
See eurypterid in American Heritage Dictionary 4
(yo͝o-rĭpˈtər-ĭd)
noun
Any of various large, segmented aquatic arthropods of the order Eurypterida that existed from the Ordovician Period to the Permian Period.