- A taxonomic class inside the phylum Arthropoda — the horseshoe crabs.
- A class of Arthropoda, allied on Crustacea. It includes the trilobites, Eurypteroidea, and Limuloidea. Each one is extinct except the horseshoe crabs (Limulus) regarding the final group. See limulus.
- a small grouping of articulated creatures to which numerous values and limits happen assigned.
- utilized in some classifications; includes the requests Xiphosura and Eurypterida