Arc Oyster Shell, Undulostrea megodon

Arc Oyster Shell, Ostrea megodon. Shell collected from within the estuary of the Magdalena Bay complex, Baja California Sur, October 2018. Size: 6.7 cm (2.6 inches) x 5.6 cm (2.2 inches).

The Arc Oyster, Ostrea megodon (Hanley 1846), is a bivalve mollusk that is a member of the Ostreidae Family of True Oysters. They are known in Mexico as ostra plegadais. The Shell is crescent shaped with deeply folded margins. There are four such folds in each valve that align with the folds on the other valve. The external surface of the shell has a rough texture. The shell has a gray exterior and a white interior, with a green tint near the interior margin.  The Arc Oyster Shell  reaches a maximum of 10.3 cm (4.1 inches) in length and 7.2 cm (2.8 inches) in height.

Arc Oysters are found are found on hard substrate in the intertidal zone to depths up to 110 m (360 feet). They range from Bahia de San Bartolome, Baja California to Peru and in the Sea of Cortez from Loreto southward to Cabo San Lucas, Baja California Sur.

Synonyms include Ostrea cerrosensis, Ostrea gallus, and Undulostrea megodon.