Family: Gryphaeidae (Vyalov, 1936)
Genus: Pycnodonte (Fischer von Waldheim, 1835) aka Gryphaea (Lamarck, 1801)
Exogyra (Say, 1820)

Family: Ostreidae (Rafinesque, 1815)
Genus: Ostrea (Linnaeus, 1758)

Oysters are an immobile mollusk attaching itself to any solid object once it passes its free swimming larva phase. The shell is made up of two valves, the upper one flat (right) and the lower convex (left).

 Many of the local New Jersey Cretaceous streams boast some truly impressive fossil oyster beds. These large shells are composed of calcite, a relatively stable form of calcium carbonate (see the aragonite dissolution discussion) which is resistant to dissolution from the acidic ground water and most have retained their original shell material. (Richards et al, 1962)  lists three genus of oysters reported from New Jersey.



 3 genus of oyster are found in New Jersey.
The genus
Gryphaea is now Pycnodonte but older reference material will use



The valves on oysters are uneven and referred to as the right and left valves.
Typically, the right valve of an oyster will be shorter and less concave than the left.
An example of the right valve
on  Exogyra.




Exogyra  sp.
The genus Exogyra is distinguished from Pycnodonte and Ostrea chiefly by it's spirally
curved peak.

Monmouth County, NJ


Robert Badger with a large Exogyra shell.

MAPS Collection




Pycnodonte sp.
Like Exogyra, Pycnodonte is a large shell, but has an almost circular shell outline
and a nearly smooth shell surface.



Ostrea sp.

This is by far the smallest of the three species. Ostrea  has a weak U shape and scalloped edge.


References and notes:

Exogyra and Pycnodonte formed vast oyster beds in a continuous belt parallel to the Cretaceous coastline. This belt starts at New Jersey, runs down to Georgia, over to Texas and up to Kansas.

 Richards, H. G., 1962. The Cretaceous fossils of New Jersey (Part 2). New Jersey Dept. of Conservation and Economic Development. New Jersey Geology Survey Bulletin, vol. 61, 237 p.

Wade, B., 1926. The fauna of the Ripley Formation on Coon Creek, Tennessee. U.S. Geological Survey, Professional Paper no. 137, 272 p., 72 pl.

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