How a shell is built

The shell is made of calcium carbonate, laid down as crystals in a matrix of protein called conchiolin. The mantle secretes it at the opening, so a shell grows by adding material at the lip, not by stretching. The oldest part is the apex, the small whorls at the tip. The newest part is the edge you can see when the animal is crawling.

Break a shell the way a thrush does, on a stone, and the layers show. There is a thin outer periostracum, often brown or olive and made of protein, which protects the minerals underneath from acidic water. Under it are thicker calcareous layers. The smooth inner surface, sometimes faintly pearly, is the side the body touches. Color and pattern, bands, flecks, a plain chestnut, are laid into the shell as it grows. A healed crack leaves a scar: a ridge, or a jog in the banding, where the mantle repaired a break and then carried on.

Most land snails coil to the right. Hold the shell with the apex up and the opening facing you, and the opening is on the right. That is dextral coiling. Sinistral, left-handed shells are the normal form in some species and a rare reversal in others. The direction is set very early in development. A snail cannot reverse it later, and a left-handed animal may have trouble mating with a right-handed partner because the anatomy has to meet.

Soils short of lime produce thinner, more easily broken shells. Old mortar, chalk, and crushed eggshell in a compost heap are ordinary sources of calcium. Acid leaf litter does the opposite, which is one reason shells are often thicker on chalk than on peat. The spiral itself is close to a logarithmic spiral, a curve that keeps its proportions as it grows. That is what lets the animal add to the lip without redesigning the apex. Popular accounts sometimes call this a Fibonacci sequence. A real shell is not obliged to match one. The geometry is already interesting.