Ammonite

Length: 3cm

Width: 2cm

Ammonites earned the nickname "snakestone" due to one of the explanations for them and their form that they were once coiled-up snakes that had been turned to stone. However, ammonites were not reptiles, but were ocean-dwelling molluscs, specifically cephalopods. They had a coiled external shell similar to that of the modern nautilus. The last of their coil-shelled kind went extinct 66 million years ago. 

Born with tiny shells, as they grew, they built new chambers onto it. They would move their entire body into a new chamber and seal off their old and now too-small living quarters with walls known as septa.