FOSSIL INVERTEBRATES/Bivalves 371 Figure Details of shell formation in a generalized bivalve (A) Marked comarginal growth lines on the shell surface (B) Section through the shell along the line indicated in (A) show ing the arrangement of growth lines within the shell (C) The relationship between the shell and the underlying mantle edge (close up details of the circled area in (B)) i, inner mantle fold; m, middle mantle fold; o, outer mantle fold calcitic, with aragonite being confined to the sites of muscle attachment and the ligament Molluscan shell is immensely strong, in fact often much stronger than vertebrate bone There are a number of different microstructures (Figure 3), each with different mechanical properties, and most shells are made up of two or three arranged in different layers Different taxa show different arrangements and these are considered to be of phylogenetic significance It is apparent that the earliest bivalves were wholly aragonitic and chiefly composed of nacre (Figure 3A), and that subsequent evolution has produced the wide array of microstructural arrangements seen today The effect of differing crystal sizes, amount of organic material, and polymorph used has affected the preservation potential of different taxa; many of the Palaeozoic and Mesozoic taxa that were originally aragonitic are either preserved as internal moulds or are replaced by calcite Details of the internal features of the shell are shown in Figure The hinge plate is situated dorsally and houses the ligament and teeth The ligament is an elastic, partially calcified layer that provides a very energy-efficient opening mechanism During valve closure, energy is stored in the ligament as it is flexed by the contracted adductor muscle(s) (Figure 4C) When the muscle is relaxed, the ligament springs the valves apart causing them to gape This passive valve opening mechanism is the reason why many fossil bivalves are found in a disarticulated state Although the ligament itself is seldom preserved, its position may be inferred from the presence of the ligament pits in which it is anchored (Figures and 5) Most bivalves have teeth on the hinge plate which fit into corresponding sockets on the opposite valve and function to keep the valves in perfect alignment Both ligamenture and dentition vary markedly amongst higher taxa of bivalves, and both are often used as informative characters in establishing phylogenies Some of the range of hinge plate architectures is shown in Figure A number of attachment scars mark the locations where muscles are anchored to the shell The most significant of these are the adductor scars (Figures 4A and 4B) If the adductor scars are paired (i.e., dimyarian), they occur posteriorly and anteriorly If an animal is monomyarian, the single muscle (the posterior) occupies a more central position In many taxa, there is a thin pallial line running around the shell a small distance from the ventral edges that marks the attachment of the mantle to the shell In infaunal taxa, where the posterior mantle has been fused and elongated to form siphons, the pallial line is inflected forming the pallial sinus The sinus represents the space into which the siphons are withdrawn when the valves are shut Other muscle attachment scars may be more or less apparent, including the insertions of the pedal musculature (particularly in burrowers and byssate taxa) Soft Part Anatomy Bivalves are laterally compressed and, unlike most molluscs, there is no head or radula The internal organs are enclosed by the two mantle lobes that are joined dorsally (Figure 4C) The chief function of the mantle is to secrete the shells, but the ventral edges of each mantle lobe are differentiated into three folds (Figure 2C), only the outermost of which is directly concerned with shell manufacture The innermost fold controls water flow into and out of the mantle cavity, whilst the middle fold has sensory capability In several bivalve groups (such as scallops), the middle fold is well developed with tentacles and eyes In some taxa, the mantle is extended posteriorly and fused to form a pair of siphons through which water is directed into (inhalant) and out of (exhalant) the mantle cavity