370 FOSSIL INVERTEBRATES/Bivalves class are too long lived and too facies specific to be of any great value General Morphology As the name implies, bivalves comprise two calcareous valves These are arranged laterally (left and right), are joined dorsally by a partially calcified elastic ligament, and enclose the soft tissue Each valve has clearly differentiated posterior and anterior features, i.e., inequilateral The primitive arrangement, retained by most bivalves, was to have a plane of symmetry parallel to the commissure (the join between the two valves), resulting in valves which are mirror images of one another (i.e., equivalve) Although this symmetry is found in virtually all bivalves which live with the commissural valve perpendicular to the substrate surface (orthothetic), it has been lost in those which have adopted a pleurothetic habit where they lie on one valve (e.g., oysters, scallops) In these cases, there is a tendency for the two valves to become dissimilar (i.e., inequivalve), typically with the underlying valve becoming more bowl-like and the ‘upper’ one more reduced like a lid Shell Morphology All bivalves possess a pair of shells which may be shut to provide protection from both environmental stresses (e.g., desiccation in the intertidal habitat) and the threat of predation Most shells are reasonably robust, which has provided the class with a generally excellent fossil record Although shell morphology in bivalves is very variable and intimately linked to their life habits (see below), all shells are simple modifications of the basic shell secretion model used by all shelled molluscs The shell is secreted by the mantle lobes and grows by marginal accretion, as evidenced by the growth lines on the surface of the valve (Figures 1A and 2A) These growth lines are particularly marked in bivalves from intertidal and shallow temperate habitats, where the animals experience pronounced seasonality and largely stop growing during the winter months Bivalves which experience more equable conditions not show such obvious or regular patterns Inspection of the growth lines in sectioned valves shows that, although most shell material is added ventrally, the shell is also thickened during growth (Figure 2B), demonstrating that the entire mantle surface is responsible for adding material The outermost part of the shell is an organic layer called the periostracum secreted at the mantle edge (Figure 2C) The thickness of the periostracum varies between taxa, from less than mm in oysters and Figure (A) Mercenaria mercenaria, a shallow burrowing bi valve from the Pliocene of Florida Note the prominent annual growth bands (B) Pecten maxima, a free living epifaunal scallop from the Holocene Atlantic scallops to several hundred micrometres in some mussels In many cases, the periostracum is lost by abrasion and decay during the life of the animal, particularly on the older parts of the shell, and there is no real prospect of it being preserved in any but the most exceptional circumstances The primary function of the periostracum is to act as the template on which the calcareous part of the shell is deposited, but it may also provide protection from both corrosive waters and predators that dissolve the shell It is particularly noticeable that freshwater bivalves have very thick periostraca The main part of the shell, however, is calcareous It is in effect a ceramic made up of calcium carbonate crystals in an organic matrix (the latter accounting for