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.