ammonite

noun

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

   noun.object

   Synonyms ammonoid

   Source: WordNet 3.0

2. 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 <i>serpent stone</i>, <i>snake stone</i>, and <i>cornu Ammonis</i>.

   noun

   Source: webster1913