ammonite Meaning, Definition & Usage

  1. noun one of the coiled chambered fossil shells of extinct mollusks
    ammonoid.

WordNet

Am"mon*ite noun
Etymology
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.
Definitions
  1. (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.

Webster 1913