As part of the Mollusc family, Pelecypods include oysters, clams, mussels, scallops, and cockles, and they have been found in some of the oldest marine rocks known while are also found today in rivers and seas.
The clam is another example of a long surviving species which has made it through the millennia to modern times. Dating to the Jurassic Period, some 150 million years ago.
Fossil clam from Florida. This guy has some calcite crystals growing on the outside as well as in the opening. It's so well preserved you can see the tiny ridges at the end of its shell. It almost looks like the clam is a mouth, and the calcite make up a bunch of little teeth.