While visiting Sanibel Island on a cold New Year's Day in 2011, I observed many small- and medium-sized horseshoe crabs, Limulus polyphemus, washed ashore in rich beach drift. I did not see any large specimens, but since I got to the island in the late morning, other people may have collected large specimens by that time. All of the crabs and other organisms in beach drift were dead.
Dead horseshoe crab on shoreline behind Whitney Lab.
Never seen this washed up on the beach before.
Merostomata is the name given to a grouping of the extinct Eurypterida (sea scorpions) and the Xiphosura (horseshoe crabs). The term was originally used by James Dwight Dana to refer to Xiphosura only, but was emended by Henry Woodward to cover both groups.