he palatal wall: the outer free wall of the final whorl of the shell.
The aperture is descending or deflected, when it does not follow the spiral of the shell, but turns downwards (such as in Helix). Sometimes it departs from contact with the preceding whorl (as frequently in Cylindrella).
The shape of the aperture in a gastropod shell can be:
- auriform, ear-shaped, as in Auriculella* bean - for example: Aerotrochus perdepressa
- circular, rotundate, orbicular
- claw-shaped aperture - Some Melongenidae have a claw-shaped aperture
- crescent-shaped - examples: Hyalosagda arboreoides, Dialeuca conspersula
- distorted aperture - Personidae has a distorted aperture
- linear, narrow. Cypraea, Conus
- longitudinal, when its greatest diameter is parallel with the axis of volution
- lunate, semilunar, semicircular or half-moon  Nerita
- transverse, when its greatest diameter is at a right angle to the axis of volution
- oblique, when the greatest diameter is oblique to the axis of volution
- oblong, much longer than it is wide, rounded above and below
- oval or teardrop - examples: Geomelania minor, Urocoptis ambigua
- ovate, egg-shaped
- patulous, when dilated and then compressed, when diminished at its entrance
- quadrate, roughly square or rectangular, as in Architectonica
- rounded, the circle slightly interrupted; example: Valvata sincera. Consequently, there is no siphonal canal but simply an opening for respiration.
- with the top angle acute - examples: Leptopeas micrum, Varicella griffithii
- triangular, as in Janthina
- without an aperture
Conus betulinus has a long and narrow aperture.
Indrella ampulla has an oval aperture.
Valvata sincera has a rounded aperture.
Theodoxus danubialis has a semicircular aperture.
Some shells, like this Patella vulgata, have no aperture.
The shells of juveniles in some species (especially some families of land snails) have a simple aperture with a sharp edge, but after reaching adult size the aperture of the shell finally acquires adult characters, consisting of a thickened, reflected, inflected or lipped edge, which is sometimes more or less contracted by inflected calcareous projections known as teeth. These teeth may be outer lip teeth, columellar teeth or parietal teeth.
Folds or plicae are named by their position in the aperture, as follows. The numbers refer to those in the diagram shown opposite:
Folds or plicae
- upper palatal
- lower palatal
Lamellae are named as follows:
- angular and twin
- "Aperture shape Bean". accessed 3 January 2011.
- "Aperture shape Crescent". accessed 3 January 2011
- "Aperture shape Half-moon". accessed 3 January 2011.
- "Aperture shape Oval or teardrop". accessed 3 January 2011.
- "Aperture shape Round". accessed 3 January 2011.
- "Aperture shape Taller than wide, top angle acute". accessed 3 January 2011.
- Pilsbry H. A. & Cooke C. M. 1918-1920. Manual of Conchology. Second series: Pulmonata. Volume 25. Pupillidae (Gastrocoptinae, Vertigininae). Philadelphia. Page vii.