|Reconstruction of Eurypterus (Eurypterida)|
|Five views of Limulus polyphemus (Xiphosura)|
Merostomata is a name given to the now extinct sea scorpions (Eurypterida) and the horseshoe crabs ((Xiphosura). There are four known species of horsehoe crabs still alive. James Dwight Dana introduced the name to refer to the horseshoe crabs only. Henry Woodward later extended it to refer to both taxa.