Also known as a "living fossil," These molts are from the Atlantic Horseshoe Crab and can be found all over the coastal areas of the United States. Horseshoe crabs have existed (and still do) on the planet virtually unchanged for over 400 Million Years! As they grow, they shed their exoskeletons or "molts" (akin to a snakeskin) which is what these delicate specimens are. They are not shells, its more like skin. As you can see from the photos below, some even have the legs and inner structure still intact!