Lucy, Laurent and I found this little horseshoe crab shell in Wellfleet Harbor.

Fun horseshoe facts I found online:

This is a crab that isn't a crab! It is distantly related to spiders and scorpions. To avoid confusion, let's call it by its correct name, Limulus polyphemus. (Pronounced Lim-u-lus poly-feem-us)

The Limulus is a "living fossil" whose origin dates back to Triassic times, over 200 million years ago, a time when the first dinosaurs and primitive mammals appeared. Few other well-known animals can claim that record. It is not easy to be a living fossil. To apply for this category, that animal can only have, at most, a few close relatives. The body form should not have changed over the years. A true living fossil's family tree must be tens, or preferably hundreds, of millions of years old.

Horseshoe crabs grow by molting and emerge 25 percent larger with each molt. After 16 molts (usually between 9 and 12 years) they will be fully grown adults.

http://seagrant.gso.uri.edu/factsheets/horseshoe_crab.html

http://www.beach-net.com/horseshoe/Bayhorsecrab.html