Exogyra Costata
Exogyra is an instantly recognizable genus of oysters from the Late Cretaceous found all over the South-Eastern US. Also called “ram’s horn” oysters, their left shell is coiled at the hinge, easily seen when viewed from the side, and have easily identifiable right (flat) shells. These are quite common in the Peedee formation, which is where the specimens pictured here originate from. Exogyra costata is differentiated from other species of Exogyra by the raised ridges that run from the base of the left shell around to the coil.
Size: 3cm - 12cm (2” - 5”)
Scale: 1cm
Age: Late Cretaceous
Left Shell
The left shell (the living chamber) of the oyster is characteristically coiled towards the hinge giving the Exogyra its distinct shape. As noted above, the ridges running along the shell are characteristic of Exogyra costata.
Right Shell
The right (or flat) shell is the one that opens to let the oyster feed, breathe, etc. It is smooth on the inside, and rough on the outside. In Exogyra it has a characteristic crescent shape.
