Opercula of Ammonites are common in many localities, especially in banks and sections of the Kimmeridge Clay; but they usually occur in broken fragments, and very rarely with their valves paired, unless sheltered within the last whirl of the shell to which they belonged. Even when thus protected the valves are generally displaced, as might be expected if we consider how slight is their union along the suture, and how great were the chances of being shifted by the contraction of the animal after death, by the pressure of external mud.

The British Museum contains several examples of Ammonites Jason, A. Brightii, A. fluctuosus, A. lingulatus, and other species with their opercula more or less shifted; and Mr. Charles Moore, of Bath, has several small shells of Ammonites planorbis from the Lower Lias, with the opercula remaining in their true position; the smallest individual is only one quarter of an inch in diameter.