The image above shows the inside of a freshwater mussel shell with all of its internal organs removed. Observe the iridescent lining of nacre (1). Near the anterior end of the valve is a raised portion called the umbo (2), which is the oldest part of the shell. The shells are held together dorsally by a spring like hinge ligament (3) that causes them to open. They are drawn together two muscles, the anterior adductor muscle (6) and the posterior adductor muscle (7). Grooves on the valves called hinge teeth allow the valves to securely interlock. In freshwater mussels there are two sets of hinge teeth, a posterior set of lateral hinge teeth (4) and an anterior set of cardinal hinge teeth (5).