A tiny little fossilized snail shell. Although usually called a turitella it is actually from a freshwater deposit, not a marine one, and the shells are those of the freshwater snail Elimia tenera. It has been preserved as agate, and then tumble polished so that you can see its internal spiral structure. It is from Wyoming. I think that this fossil will work in both 1:12 and 1:24 settings. It is discreetly glued to a polished black onyx base.
3/8 inch (0.95 cm) wide, just over 1/4 inch (0.6 cm) deep and just over 1/4 inch (0.6 cm) tall.