Although this symmetry is retained in most bivalves, the symmetry is secondarily lost in others. Note that in one of the specimens, there is close to a plane of symmetry within the valves. This asymmetry corresponds to the living habit of the beasts, and those who have lost their original bilateral symmetry between the valves commonly live with their plane of commisure not perpendicular to the sediment surface. The two halves of the bivalve shell are usually joined at the dorsal margin by a ligament which acts as a spring. The ligament may be internal, as in oysters, mussels, or scallops.
Some internal ones consist of horizontal or vertical bands which sit in grooves.
Other ligaments may be external. General terms that you will need to know are given in the next section.