Gastropod operculum

                              apertural view of Megastraea undosa, height 5 cm, diameter 7 cm.

This post concerns the operculum ("trap door") that closes the aperture of certain shells, like that of the turbinid shallow-marine gastropod Megastraea undosa (W. Wood, 1828) shown above. The reason that I picked this topic is because when beginning fossil collectors find a gastropod operculum they commonly do not have a clue as to what kind of animal made it.

Exterior side of the operculum of M. undosa. The width is 3 cm. 

Interior side of the operculum shown above.

Same specimen of Megastraea undosa as shown above but now with its operculum, which very effectively seals the aperture from predators.

Megastraea undosa is found today in central California and southward along the outer coast of Baja California, Mexico. The species is restricted to hard substrates in the intertidal zone and grazes on algae attached to rocks. The species has a fossil record extending back to the approximately 10 million years ago (i.e., late Miocene).