Source : Webster's Revised Unabridged Dictionary (1913)

Eurypteroidea \Eu*ryp`te*roi"de*a\, n. pl. [NL. See
   {Eurypteroid}.] (Paleont.)
   An extinct order of Merostomata, of which the genus
   Eurypterus is the type. They are found only in Paleozoic
   rocks. [Written also {Eurypterida}.]

Source : WordNet®

Eurypterida
     n : extinct aquatic arthropods of the Paleozoic [syn: {order
         Eurypterida}]