1.  clamshell(n = noun.animal) - the shell of a clam;
is a kind of shell

2.  clamshell(n = noun.artifact) grapple - a dredging bucket with hinges like the shell of a clam;
is a kind of dredging bucket

1.  the shell of a clam. [WordNet 1.5]

2.  a dredging bucket with hinges like the shell of a clam. [WordNet 1.5]