Xiphosura are traditionally placed in the class Merostomata . This term was originally made for the eurypterids ; now , with the Xiphosura added , it is known to be paraphyletic . Xiphosura are traditionally placed in the class Merostomata , although this term was originally intended to encompass also the eurypterids , whence it denoted what is now known to be an unnatural ( paraphyletic ) group .