Final answer:
The umbo is the oldest part of a bivalve shell, which is the initial point of growth and the part where the two shells are joined together.
Step-by-step explanation:
The oldest part of a bivalve shell is called the umbo. Bivalves, which are part of Class Bivalvia, are enclosed in a pair of shells or valves that are joined at the umbo, the initial point of shell growth. The shell is composed of different layers including an outer organic layer, a middle prismatic layer, and an inner smooth nacreous layer. Anterior and posterior adductor and abductor muscles help in the opening and closing of these shells. Examples of bivalves include clams, oysters, mussels, and scallops, all of which exhibit marine as well as freshwater lifestyles.