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 marine Chelicerata which includes horseshoe crabs and eurypterids. It includes only four living species but dozens of fossil species mainly from the Paleozoic. The class Merostomata is slowly being abandoned in favour of splitting it into two classes, Xiphosura and Eurypterida.