A fossil cephalopod shell related to the nautilus. There
are many genera and species, and all are extinct, the typical forms
having existed only in the Mesozoic age, when they were exceedingly
numerous. They differ from the nautili in having the margins of the
septa very much lobed or plaited, and the siphuncle dorsal. Also called
serpent stone, snake stone, and cornu Ammonis....
