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A purist might forgive snails for departure from the bilateral paradigm if only they honored an even more inclusive symmetry by growing right and left-handed spirals in equal numbers. But snails remain twisted and awry on this criterion as well — for right-handed shells vastly outnumber lefties, not only in the sacred conch [Turbinella pyrum] of India, but in virtually all species and groups. Right-handed shells are called "dextral," from the Latin dexter, meaning "right," and memorialized in our language by a host of prejudicial terms invented by the right-handed majority to honor their predominance. Right is dextrous, not to mention "correct" in many languages — awright buddy. The law, by the way, is droit in French and Recht in German, both meaning "right." [Also Polish "prawo"] (The language police will never regulate these essays, but we may still note, in fairness and for historical interest, that the "rights of man," noble as the sentiments may be, embody two linguistic prejudices of unfairly dominant groups.) Left-handed shells are called "sinistral," from the Latin sinister, meaning "left" — also denigrated in our languages as "sinister" or gauche, a French lefty. […] I also can’t help wondering if we didn't make our initial decision to call a snail's apex "up" because this orientation would then allow us to designate the vastly more common direction of coiling as "right."
The vast majority of forms grow a dextral shell, although a few sinistral specimens have been found in most species. For example, in Cerion, the West Indian land snail that forms the subject of my own technical research, only six sinistral specimens have ever been found, out of millions examined (while, as stated above, lefty Turbinellas in India were literally worth their weight in gold). A few species grow exclusively or predominantly sinistral shells, but related species of the same group are usually dextral. We often exact a price from these rare sinistrals by giving them names to match their apostasy—as in Busycon contrarium or Busycon perversum, the technical monikers variously awarded to the most common sinistral species of northern Atlantic waters. A few groups of species (notably the family Clansiliidae) are predominantly sinistral, but, again, all closely related lineages are dextral. In short, dextral snails greatly predominate (at a far higher frequency than human righties vs. lefties), and at all levels: individuals within a species, species within a lineage, and lineages within larger groups.
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(Stephen Jay Gould, Dinosaur in a Haystack, 16. Left Snails and Right Minds)
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