A variety of marine bivalve mollusks which penetrate solid substrata. They represent seven families and vary in the extent to which they are specialized, in the type of substrata they utilize, and in their method of boring. It is difficult to differentiate borers, burrowers, and nestlers because some species, such as Hiatella arctica, not only bore into hard limestones, calcareous sandstones, and chalks, but also burrow in peat or sand-filled crevices in hard rocks, or nestle in holdfasts of Laminaria. Undoubtedly the true borers evolved from burrowers via forms that were living in increasingly harder substrata.