What is "merostomata"

Merostomata \Mer`o*stom"a*ta\, prop. n. pl. [NL., fr. Gr. me`ros thigh + ?, -?, mouth.] (Zo["o]l.) 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.

Merostomata is a class of chelicerate arthropods that contains the extinct Eurypterida (sea scorpions) and the extant Xiphosura (horseshoe crabs). The term was originally used by James Dwight Dana to refer to Xiphosura only, but was emended by Henry Woodward to cover both groups.