The shell of a bivalve is composed of calcium carbonate, and consists of two, usually similar, parts called valves. These are joined together along one edge (the hinge line) by a flexible ligament that, usually in conjunction with interlocking “teeth” on each of the valves, forms the hinge. This arrangement allows the shell to be opened and closed without the two halves detaching. The shell is typically bilaterally symmetrical, with the hinge lying in the sagittal plane. Adult shell sizes of bivalves vary from fractions of a millimetre to over a metre in length, but the majority of species do not exceed 10 cm (4 in).
The shell is added to in two ways; the valves grow larger when more material is secreted by the mantle at the margin of the shell, and the valves themselves thicken gradually throughout the animal’s life as more calcareous matter is secreted by the mantle lobes. Although the (sometimes faint) concentric rings on the exterior of a valve are commonly described as “growth rings” or “growth lines”, a more accurate method for determining the age of a shell is by cutting a cross section through it and examining the incremental growth bands. Use of this technique has changed views on the longevity of many bivalves.
Adapted from: https://en.wikipedia.org/wiki/Bivalvia, on 2018/04/20