What is the oldest part of a clam's shell called?

The oldest portion of a clam's shell is called the beak or umbo. Clams grow their shells around themselves, continually adding new material in concentric rings, starting at the beak and moving outward.

Since clams are bivalves, as they always have two beaks, one for each half of their shell. The two shells are joined together by a strong ligament that forms a hinge close to the beak. The beak is not used for feeding, as it merely is a structure that remains in place as the clam grows throughout its life. The umbo is sometimes distinguished from the beak as being the highest point of the shell, but most of the time the beak is also the highest point of the shell or very near it, so the two are conflated.