The shell consists of three layers: An outside layer,the periostracum, made out of several layers of conchiolin.This contains the pigments which give the shell its color. An inner layer,the ostracum,made out of crystalline calcium carbonate. An innermost layer,the hipostracum,composed of calcium carbonate. The thickness of the shell depends on the age of the animal,the species and the environment.For example snails which live in marshy areas often have thin shells and snails which live in hot,dry areas have thicker shells. The oldest part of the shell is the apex.From the apex the shell grows downwards, every 360 degrees turn is a whorl.The area between the whorls is called the suture. The whorls can be turned dextral or sinistral.Most species are dextral,but in some Families the shells are coiled sinistral. If you hold the shell with the aperture towards you,it is dextral when the aperture is on the right side,sinistral when it is on the left side. The central column in the shell is called the columella.On the underside of the shell is the umbilicus,this can be closed to wide open. The last whorl of the shell is called the bodywhorl,this leads to the shellmouth or aperture.Around the mouth is the lip,this can be thick or thin and often has a different color than the rest of the shell. The surface of the shell can be smooth or grooved.The grooves can be vertical(common) or spiral.Sometimes there are raised areas between the grooves forming ribs. This can be faint or strong. Cerion yumaensis
The aperture of the shell can be closed with an operculum in the Prosobranch class.This is composed of a corneous material(except in the genus Pila where it consists of a calcified matter).