The Collaborative International Dictionary of English v.0.48:

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.]
   [1913 Webster]