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Bivalves
Bivalves (Bivalvia, Lamellibranchiata, Pelecypoda) are a common class of molluscs, which mainly thrive either attached to rocks near the shoreline, or buried in sand near
the shore. They are also commonly known
as clams. A few, such as Pecten, are able to move about by opening and shutting their valves quickly. Most attach themselves to one rock (for example, mussels), while a
few use their foot to push themselves
across the sand, or up and down in
their burrow (e.g. razor shells).
Bivalves were rare in the Lower Palaeozoic Era, and even in the Carboniferous Period they were not common, although freshwater mussels are very important
for relating various rocks formed on land.
Bivalves were much more common in the Mesozoic Era, when the number of species increased considerably. By the Tertiary Period, bivalves and gastropods were even more numerous and many beds are identified using them.
(Below) The main characteristics of a bivalve viewed from above (top) and the side (bottom).
Dorsal margin (hinge)
Front
Growth line
Back
Symmetrical between valves from dorsal margin
Ventral margin
Umbo
Teeth and sockets
Scars left by muscle attachments
Scars left by muscle attachments
Siphons
Foot
Ventral margin
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