horseshoe crabs

Limulidae is the only recent family of the order Xiphosurida and contains all the 4 living species known of the class Xiphosura, formerly called Merostomata. The best known species is the horseshoe crab (Limulus polyphemus), whose ancestors can first be seen in the Devonian period's fossil record. They are found along the northwestern Atlantic coast and in the Gulf of Mexico.