AbstractA study of the isthmus and ligament in unionid mussels was undertaken employing methods specifically designed to preserve the in vivo relationships of these interrelated structures. Serial sections of the hard and soft tissues were used for two‐dimensional analysis. From these, a tridimensional computerized reconstruction was developed. Special dissections of the undisturbed isthmus were also utilized. By using such methodologies, a new description of the ligament has been developed employing such terminology as the foliated ligament and the posterior folding laminae. Similarly, for the isthmus, an anterior lyre, a pallial crest, a pallial peduncle, and a posterior lyre are described. Such entities are both morphologically and physiologically related to the shell and ligamen