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In geology, the terms sinistral and dextral refer to the horizontal component of the movement of blocks on either side of a fault or the sense of movement within a shear zone. These are terms of relative direction, as the movement of the blocks is described relative to each other when viewed from above.
The shell is shown in its entirety, with its aperture facing the viewer, and the apex at the top. If the aperture is on the right side when viewed like this, then the shell-coiling is "right-handed" or dextral; if the aperture is on the left side when viewed like this, the shell has "left-handed" or sinistral shell-coiling.
The opposite is sinistral (Latin: sinister, left). This is consistent with the terms for right-handed screws in engineering and physics. Most species of sea snail are dextral. Within a typically dextral species, rare individuals may develop sinistral coiling. In religious usage, the shankha (sacred conch shell) is displayed spire downwards.
Snails in the family Physidae have shells that are sinistral, which means that if the shell is held with the spire pointing up and the aperture facing the observer, the aperture is on the left-hand side. The shells of Physella species have a long and large aperture, a pointed spire, and no operculum. The shells are thin and corneous and rather ...
The shell of this species, like all planorbids is sinistral in coiling, but is carried upside down and thus appears to be dextral. The shell of Indoplanorbis exustus is discoid with rapidly increasing whorls. [5] Each whorl is higher than it is wide. [5] The width of the shell is 5 [8] –25 mm. [5] The height of the shell is 4.5 [8] –13 mm. [5]
A few of the species in this genus are unusual in that specimens in those species always have left-handed "sinistral" coiling in their shells, as shown in the specimen on the right. The rest of the species in the genus are right-handed or "dextral" in the shell coiling, as is usually the case in the great majority of gastropods.
Turbinella pyrum, common names the chank shell, sacred chank or chank, also known as the divine conch or referred to simply as a conch, is a species of very large sea snail with a gill and an operculum, a marine gastropod mollusk in the family Turbinellidae.
In English, the shell of this species is known as the "divine conch" or the "sacred chank". It may also be simply called a "chank" or conch. There are two forms of the shanka: a more common form that is "right-turning" or dextral in pattern, and a very rarely encountered form of reverse coiling or "left-turning" or sinistral. [9]