The clamshell design was a style of communicator named for its resemblance to the shell of a clam in the way that it shut. They were mainly used by Starfleet in the 22nd and 23rd centuries. By the late 24th century, they had been replaced by combadges.
In 2380, D'Vana Tendi hoped that a mysterious cargo turned out to be communicators made with the original clamshell design. (LD: "Terminal Provocations")