A depressed or raised marking on the interior of a shell, usually of a brachiopod (Brachiopoda) or bivalve (Bivalvia), which was the point of attachment of a muscle. In articulate brachiopods (Articulata) there are commonly two pairs of muscle scars occurring in the floor of the dorsal and ventral valves, one pair formed by the adductor muscles that close the shell and the second pair by the diductors, that open it. There are additional muscles present in inarticulate brachiopods (Inarticulata) and these also leave scars. In bivalves there are commonly two muscles present (dimyarian); these are the adductor muscles that close the shell against the opening moment of the ligament. In some species the scars are equal in size (isomyarian), in others unequal (anisomyarian) and the smaller scar is always the anterior one. Sometimes the anterior muscle is lost and the remaining muscle may increase in size; in this condition the bivalve is ‘monomyarian’. Connecting the two scars on the interior of the valve there is the pallial line, a linear depression marking the inner margin of the thickened mantle edges that may be marked by an inward deflection at the posterior part, the pallial sinus. This defines the space for the retraction of the siphons and the place of attachment of the siphonal retractor muscles.