Extinct mollusc genus of the Oyster family, commonly found in shallow-water marine deposits of the Jurassic to Cretaceous periods (dating around 65-190 million years old). Exogyra characteristically had a very thick shell which attained massive proportions. The beak of each valve is spiral. The left valve is spirally twisted, deeply convex, usually strongly marked by radial folds of the surface whereas the right valve is flatter, concave and much smaller and contains a surface that is either smooth or marked by faint radiating lines. A distinctive longitudal pattern of ribbing is highly developed in the left valve. The genus possessed inequilateral, twisted shells resembling somewhat the form of the young plicate shells of the modern edible oyster though the shell of Exogyra is larger and more convex.

Exogyra upatoiensis
Exogyra ponderosa

References and Further Reading

  • Funk and Wagnalls Encyclopedia ©1950
  • Encyclopedia Britannica Micropedia ©1984
  • Exogyra collection of C. Newsom, Collected from Texas, Alabama, New Jersey, Utah and Nevada
  • Paleontology of Kentucky
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