1) Animal order 2) Order xiphosura 
Suborder Xiphosurida Xiphosura were traditionally placed in the class Merostomata, although this term was intended to encompass also the eurypterids, whence it denoted what is now known to be an unnatural (paraphyletic) group. Although the name Merostomata is still seen in textbooks, without reference to the Eurypterida, some have urged that this ...
• (n. pl.) See Xiphura.
Xiph`o·su'ra noun plural See Xiphura . 
Xiphosura are the king-crabs order of Arachnida. They are aquatic animals with gill-books.
[n] - horseshoe crabs and extinct forms
an Order of marine chelicerates that include the Horseshoe crabs.
order Xiphosura noun horseshoe crabs and extinct forms