1) Mollusc shell 
a line joining the points of maximum curvature along the hinge of a fold.
Hinge line is term applied to the shell of a bivalve mollusk. It is an imaginary longitudinal line along the dorsal edge of the shell, where the two valves hinge or articulate. The hinge line can easily be perceived in these images of a mussel shell and an ark shell. The hinge teeth, structures which control the articulation of the valves, are oft...