The Collaborative International Dictionary of English v.0.48:

Ammonite \Am"mon*ite\, n. [L. cornu Ammonis born of Ammon; L.
   Ammon, Gr. ? an appellation of Jupiter, as represented with
   the horns of a ram. It was originally the name of an.
   Egyptian god, Amun.] (Paleon.)
   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.
   [1913 Webster]