Shape
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.
- semicircular
- with the top angle acute - examples: Leptopeas micrum, Varicella griffithii
- triangular, as in Janthina.
- without an aperture
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Conus betulinus has a long and narrow aperture
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Indrella ampulla has an oval aperture
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Valvata sincera has a rounded aperture.
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rounded aperture of Valvata piscinalis can be covered by an operculum
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Theodoxus danubialis has a semicircular aperture.
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Some shells, like this Patella vulgata, have no aperture.
Read more about this topic: Aperture (mollusc)
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