The two valves are held together in life by the long, straight, developed hinge occupying the anterior end of the dorsal margin (Fig 1). Note the narrow, straight, proteinaceous ligament extending for the length of the hinge. Hinge teeth are weakly developed in mussels. The inconspicuous umbos are located at the anterior end in Mytilus. The valve is covered by the conspicuous, dark yellowish brown or black, proteinaceous periostracum. This outermost layer of the shell can be seen folded over the ventral edge of the valve in fresh specimens. The three shell layers are, from outside in, the organic periostracum, calcareous prismatic layer, and calcareous, nacreous. The valves are often eroded so that the chalky white calcareous prismatic layer shows through the dark periostracum.