PHYLUM MOLLUSCA -- CLASS BIVALVIA
The Bivalvia (AKA Pelecypoda) are those molluscs with a bivalved shell. Unlike the brachiopods, the shell of most comprises valves that are mirror images of each other. The individual valves themselves do not show any symmetry.
Bivalves that do show symmetry of the two valves are known colloquially and collectively as "clams". The first picture shows internal molds of a couple of such clams, which usually do occur as molds since the shells are made of the unstable form of calcium carbonate called aragonite. The specimen on the left is shown parallel to the hinge (at the top) and the symmetry is apparent. That on the right shows a single valve, and its asymmetry is also evident. Both are from Lower Cretaceous rocks near Blanco, TX. The scale bar is 1cm. (Photo by B. Carter.)
Some Bivalvia are asymmetric regardless of the direction from which they are viewed. In the case of oysters the asymmetry is obvious. The picture below shows Exogyra ponderosa from Upper Cretaceous rocks near Columbus, GA. The larger valve on the left is strongly convex toward the observer. The smaller valve on the right is nearly flat, and fits down inside the larger one. Oysters make their shells almost entirely out of calcite rather than aragonite, so the valves are usually preserved intact, not as molds. The scale bar in the photo is 1cm. (Photo by B. Carter.)
Scallops are another group of asymmetric bivalves that are sometimes, unfortunately, almost symmetrical like brachiopods are. The photo below is of a Middle Eocene scallop from near Americus, GA (Chlamys cawcawensis). At first glance it appears that the valve shown is bilaterally symmetric, but closer examination shows this not to be so. Notice particularly that the projections at the bottom of the photo (the "ears") have subtly different shapes, as do the left and right edges. The valve not shown is slightly larger and more convex than this one, so the shell is asymmetric. The scale bar is 1cm. (Photo by B. Carter.)
