The Collaborative International Dictionary of English v.0.48:

Merostomata \Mer`o*stom"a*ta\, prop. n. pl. [NL., fr. Gr. me`ros
   thigh + ?, -?, mouth.] (Zool.)
   A class of Arthropoda, allied to the Crustacea. It includes
   the trilobites, Eurypteroidea, and Limuloidea. All are
   extinct except the horseshoe crabs (Limulus) of the last
   group. See Limulus.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Merostomata
    n 1: used in some classifications; includes the orders Xiphosura
         and Eurypterida [syn: Merostomata, class Merostomata]