These two distinct substances have been described by many writers as though, composing the same mass, they were of one substance; but the difference may very easily be explained. The _true ligament_ is external, being fixed on the edge of one valve behind the umbones, and passing over in an arch to the corresponding edge of the other, very correctly retaining the name of _ligament_, because it serves the purpose of binding the two together. The thick, elastic substance, which Mr. Gray names the _cartilage_, is sometimes found in connexion with the ligament, so as to form one mass with it, although it is always separable and placed within it: it is sometimes placed quite within the shell, and separated from the ligament, in a pit or hollow formed for its reception in the hinge lamina, near the centre. It is found in both valves, and being elastic, the portion in one valve presses against that in the other, so as to keep the valves apart, unless voluntarily closed by the adductor muscles of the animal. The ligament is sometimes spread over an external area, as in Arca, while the cartilage is placed in several grooves of the same area, beneath the outer covering. _Hinge lamina, Teeth and Fulcrum of the Ligament._ In a great variety of cases, there is a thickening of the substance of the shell within, under the dorsal margin; this is named the hinge lamina. It is sometimes merely callous; but in many cases it has raised _teeth_ in both valves, those in one valve entering into corresponding cavities in the other. Those which are placed immediately below the umbones, and seem to take their rise from beneath them, are called _cardinal teeth_; those at a distance from the umbones, which are seen to lie along the upper margin of the shell are named _lateral teeth_. When the cardinal teeth terminate in a double point, which is not unfrequently the case, they are said to be _bifid_. The lateral teeth, in various species, are distinguished as terminating _near_ to, or at a _distance_ from the umbones. In the Nuculæ and Arcæ there is a row of teeth placed across the hinge lamina. In which case, the lateral cannot be distinguished from the cardinal teeth. _Muscular Impression._ [Illustration: Fig. 74. _a, anterior; p, posterior; m i, muscular impressions._]