Suture (gastropod)
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The suture is an external feature of the shell of a snail, a shelled gastropod mollusk. This feature is found in nearly all spirally-coiled gastropod shells, whether they are of land, freshwater or marine species.
To explain what the suture is, it is necessary to first explain that the shell of nearly all snails consists, in effect, of a tube of increasing diameter, closed at the small end, and spirally wrapped around a central axis.
Each complete rotation of this spirally-arranged tube is called a whorl. The whorls of a snail shell usually overlap one another forming a spire. Where the whorls overlap, there is usually a clear indentation. This indentation forms a visible line which reaches from the apex to the body whorl; this line is the suture.
Details of the suture are often useful in discriminating one species from another. The suture also provides a sort of geographic marker from which one can refer to the positioning of patterning or sculpture where that is relevant: for example some species have a darker or lighter subsutural band on the shell.
