White Bearded Ark
Barbatia candida Helbling, 1771
The White Bearded Ark is originally white, and with a yellow/brown periostracum, a hairy-looking material which covers the shell. (The name implies that the periostracum is white, which it isn't.) The ribbed shell may attain a length of as much as 3 inches and is found on the shore from North Carolina to Texas, as well as on beaches in the West Indies. The living shell, using its threadlike byssus, attaches itself to rocks in shallow water.
The hairy/bearded periostracum is worn off in this photo, and the shell is not the original white. It probably took on the color of the sand, mud, or debris in which it was lying.