374 FOSSIL INVERTEBRATES/Bivalves Figure Selected hinge plates showing some of the variety of ligament insertion and arrangement of teeth (A) Cerastoderma edule: heterodont dentition with two centrally placed cardinal teeth and two lateral teeth (B) Venus casina: heterodont (similar to Cerastoderma but with no lateral teeth) (C) Arca tetragona: taxodont dentition with numerous teeth arranged in a row; the ligament forms a chevron pattern on the broad triangular area below the umbones (D) Chlamys varia: two simple teeth with the internal ligament occupying a triangular pit below the umbones Figure Morphology and mode of life of bivalves living in or partially within soft substrates (A) Pinna nobilis Not to scale (B) Mya truncata (C) Cerastoderma edula in the adductor musculature, which (although still dimyarian) is heteromyarian, with the anterior adductor much smaller than the posterior (Figure 7D) Pleurothetic byssate bivalves are often markedly inequivalve, with the ‘lower’ valve (which in the majority of cases is the right) often larger than the other Although they are dimyarian early in ontogeny, the ‘adults’ are monomyarian, having lost the anterior muscle during ontogeny; the remaining posterior muscle is often large and centrally placed (Figure 7C) The presence of a byssus may be inferred from either a slight gape between the valves through which it passes (the byssal gape), or more obviously the byssal notch in scallops (Figure 4B)