Free Spirals in Time: The Secret Life and Curious Afterlife of Seashells by Helen Scales

expand a winglike flap on their shells that prevents them from sinking into silty mud. Another strategy used by epifaunal species is what Vermeij describes as the ‘iceberg habit’. Instead of lying on the surface they allow themselves to sink in slightly so that most, but not all, of the shell is submerged. Scallops commonly have a curved lower shell that sticks a short way into the mud.
    Shape also matters for infaunal species, those that spend their lives burrowed down into mud and sand. Among the sea snails and bivalves there are champion diggers that use their feet as spades to bury themselves completely in under a second. Some have tiny ratchets on their shells to prevent them slipping backwards, and others have smooth whorls to make sure sand and mud don’t stick to them and increase the load.
    Burrowing shells face the additional problem of being unearthed. If you’ve ever stood barefoot in lapping waves on a sandy beach, you may have noticed the sand being scoured from around your toes. When waves and currents flow around a solid object they stir sand grains into suspension and whisk them off elsewhere. To overcome this, burrowing shellsevolved spines and ribs that trap sand particles and stabilise the sediments around them. A group of typical diggers are tower shells, which look like little unicorn horns; their sculpted whorls help to hold them in place in their sandy, muddy homes and reduce the chances of being swept away.

    Back inside Raup’s imaginary museum of all shells, there is another perplexing detail that needs explaining: all the coiling shells twirl in the same direction. Suspended from their wires, the glass models have their tips pointing downwards and their apertures all open to the right. Or, seen from the top, they coil in a clockwise direction. Raup could easily have filled his museum with shells that twist the other way, or perhaps made two giant rooms that were mirror images of each other. But he didn’t, and for good reason.
    Take a look at any real, spiralling shell and see which way it turns. Go and find that seashell sitting on a bookcase, or pick up a snail from your garden or local park; your shell almost certainly coils to the right. There is a smattering of species that always coil to the left, and occasionally sinistral oddities will occur in a right-coiling species, but currently the natural world favours righties over lefties. More than nine out of ten coiled shells today are dextral (curiously, a similar proportion of people are right-handed).
    Shell collectors go crazy for rare sinistral specimens, so much so that over the years clandestine trades have prospered in fake lefties. Some are right-coiling shells that have undergone a bizarre molluscan version of plastic surgery, with some bits cut off and others glued back on; X-rays show their insides are in fact dextral. There are also true left-coiling shells that masquerade as something more special. Around the world, Hindus and Buddhists are summoned to prayer by the call of sacred conch-shell trumpets, known as shankh in Sanskrit. These are made from a large species ofIndian Ocean gastropod, known in English as a chank shell, which normally coils to the right. Rare left-coiling specimens are highly revered, and are referred to variously as dakshinavarti shankh or sri lakshmi shankh . Their anticlockwise whorls are said to mirror the passage of the stars and sun across the heavens, and the curly hair and twisting bellybutton of the Buddha. Unscrupulous shell-traders make counterfeit sri lakshmi shankh shells from a different species, the Lightning Whelk, which lives in the Gulf of Mexico and normally coils to the left.
    A famous left-handed shell was drawn by Rembrandt. He portrayed a Marbled Cone Snail which, like most of the poisonous cone snails, naturally coils to the right. Art historians speculate that Rembrandt hadn’t made a mistake, as many early shell illustrators did. Failing to appreciate the significance of