eu·ryp·ter·id 
(y-rĭptər-ĭd)
(y-rĭptər-ĭd)n.
Any of various large segmented aquatic arthropods of the order Eurypterida that existed from the Ordovician Period to the Permian Period, having a long tail often with a spinelike appendage at the tip. Also called sea scorpion.
[From New Latin Eurypterida, order name, from Eurypterus, genus name : EURY- + Greek pteron, wing; see -PTER.]