Ammonite

Ammonites are the fossils of the ammonoids, an extinct subclass of cephalopod molluscs, nearer kin to octopuses, squid and cuttlefish than to the nautilus whose coiled shell they resemble. They first appeared in the Early Devonian, around 410 million years ago, and the last died out at or soon after the mass extinction of 66 million years ago. More than 10,000 species have been described, and they serve geologists as index fossils, tying the rock layers they lie in to particular periods. What survives is mostly the shell, usually a flat spiral divided into chambers. Pliny called them horns of Ammon, after the ram-horned Egyptian god, and the name ammonite comes from that. In medieval Europe they were taken for snakes turned to stone, called snakestones or serpentstones, linked with Hilda of Whitby and Saint Patrick, and credited with healing and oracular powers; traders sometimes carved a snake's head on them. Others called them salagrana and used them to ward off witches. Hindus hold the ammonites of the Gandaki River in Nepal, called Shaligrams, to be a manifestation of Vishnu. Cunningham records that ammonites, called Draconites in the Middle Ages, were thought to be stones from dragons' heads and were bound to the left arm for protection; fossils of every kind are also worn to lengthen life.