ammonite

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.

Source: Webster's Revised Unabridged Dictionary (1913), United States public domain. See methodology for full licensing detail. Word validity for game play may differ from any official game dictionary.