Bivalve oyster shell
Zig-zag growth lines
Here is an oyster with two shells that opened and closed. As you can see, the shell edges have a zig-zag pattern and can be mistaken for jaws. This pattern made the shells robust. The parallel lines above and below the ‘jaws’ are growth lines charting the life of the shell. Oysters are distinctive in that their shell is composed of many fine layers.
Find out more about bivalves on the molluscs pages.