hinge line
From ZooTerms (Dictionary of Invertebrate Zoology)
hinge line: 1. (Arthropoda: Crustacea) The middorsal line of junction between two valves of the carapace permitting movement between them.
2. (Brachiopoda/Mollusca: Bivalvia) The external line of meeting of the brachial and pedicle valves; movement of the shells occurs here; hinge axis.
