The ligament of Pecten maximus consists of two layers, the outer extending along the dorsal margin of the valves, and the inner situated between the valves in the region of the umbo. The former appears fibrous and is somewhat similar to the outer layer of other ligaments. The inner layer consists of three parts, a large central non-calcified structure and two lateral calcified regions attaching the former to the valves. The central region is characteristic of the ligament of P. maximus and associated species, this layer being uniformly calcified in the ligament of most bivalves.
When the valves are closed the outer layer is subjected to tensile stress and the inner to compression, and the force so derived tends to open the valves. This force and that required just to close the shell are expressed as the opening and closing moments of the ligament respectively, and these have been determined for various lamellibranchs.
The exact conditions of the opening and closing of the valves have been observed by drawing the stress-strain curves for the intact ligament, plotting the applied moment against the angle of gape. The loading and unloading curves so produced describe a hysteresis loop. The area enclosed in that of Pecten (or Chlamys) is markedly less than that of other lamellibranchs and indicates the greater efficiency of the ligament of the former.
The mechanical properties of the isolated central region of the inner layer of the ligament of Pecten were investigated and showed similar properties to those of the intact ligament. The modulus of elasticity in compression of this region of the ligament of Pecten is approximately one-seventh of that of the inner layer of Ostrea or Lutraria, which are calcified structures. The ligament of the latter two bivalves has a greater opening moment but lower efficiency than that of Pecten. The characteristics of the ligament of Pecten are probably due to the central non-calcified region of the inner layer. The significance of these properties in regard to the swimming habit of Pecten is briefly discussed.