Mollusks are the most endangered group of animals in the U.S.
The holes in these shells were not naturally caused. They were put there by humans to make buttons, in the late nineteenth and early twentieth centuries. Shells of mollusks are very beautiful on the inside, composed of "mother-of-pearl," a shiny opalescent layer.
Large freshwater mussels, abundant in the Southeastern United States, were harvested, the animals killed, the meat discarded and buttons made from their shells. As the shell button industry was not regulated for much of its existence, freshwater mussels were over-harvested. In the 1940s, plastic replaced shell as the material for buttons, but freshwater mussels remain the most endangered group of animals in the United States, period.