What causes the holes in bivalve shells?

(What causes the holes in shells? I collected shells on a South Carolina beach last week. Some are solid (halves) and some have a single very uniform hole on one end. Thanks!)

Such holes are caused by predatory snails, usually in the family Naticidae (commonly called "moon snails"). These snails regularly feed on bivalve mollusks like clams, mussels and oysters. They use their radula, a sort of tongue covered with tiny toothlike projections, to rasp a hole in the bivalve shell, allowing them to insert their proboscis and feed on the soft parts of the bivalve. (Answer by M. Paul Monfils via the Forum)