We counted 179 of these. 55 were single, the rest were mating. 51 makes, 4 females of the singles.
Last night in Rhode Island, we counted over 100 crabs coming ashore to mate. We found many more males than females, and the males were mostly smaller (18-23cm wide) than the females (22-25cm wide). Many males tried to push others off the females, while the females lay half buried in the sand laying eggs.
There were many dead ones caught up in the rip-rap armoring the shoreline here.
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.