This image shows the inside of a freshwater mussel shell with its iridescent lining of nacre (mother of pearl) secreted by the mantle.   Near the anterior end of the valve is a raised portion called the umbo, which is the oldest part of the shell.   Bivalve shells are held together by a dorsal hinge ligament that causes the valves to open ventrally.  The valves are drawn together by a pair of anterior and posterior adductor muscles.  Grooves on the valves called hinge teeth  help to provide a mechanism for securely interlocking the valves together.  In freshwater mussels there are two sets of hinge teeth,  a posterior set of lateral hinge teeth and an anterior set of cardinal hinge teeth.