ammonite Definition

ammonite
noun

1. zool.
An extinct marine cephalopod mollusc with a flat, tightly coiled shell, widespread during the Mesozoic era.
2. geol.
The fossilized remains, especially the shell, of this animal.

Etymology: 18c: from Latin Cornu Ammonis horn of Ammon, apparently because of the fossil's tightly spiralled appearance.