A vocabulary of landscape — a landscape of words
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ammonite
The coiled, chambered shell of an extinct marine cephalopod. The animal lived in the last and largest chamber and controlled its buoyancy in the sealed ones behind it through a tube called the siphuncle. Ammonites had a run of something like 340 million years, which is a longer stretch than most groups of animals ever get. Because species turned over quickly and drifted widely, they are among the best index fossils there are.
Etymology
Latin, and specifically Pliny the Elder, who in Natural History lists Hammonis cornu, the horn of Ammon, among the most sacred stones of Aethiopia — gold-colored, shaped like a ram’s horn, and said to bring prophetic dreams. Renaissance naturalists picked Pliny’s name back up, and in 1732 Johann Philipp Breyne fixed Ammonites as the formal term for the group.
Notes
In medieval England they were snakestones — coiled serpents turned to rock, credited to Saint Hilda of Whitby, who was said to have cleared the headland of them. Dealers carved snake heads onto the open end and sold them to pilgrims.