Land snails from Mexico in the family Polygyridae. Many land snails have barriers ("teeth") in their shell apertures, which help keep out predators, e.g., beetles. Scanning electron microscope studies show that the surface of the barriers are not smooth, but consist of the jagged edges of calcium carbonate crystals, with the crystal edges directed outward.
John B. Burch (photographer; copyright holder), Mollusk Division, Museum of Zoology, University of Michigan-Ann Arbor
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