Snails are not the only animals to gnaw mineral substances for skeletal material. Many animals (deer, rabbits, porcupines, etc.) gnaw bones for their calcium phosphate; this is one of the reasons why bones do not last long when exposed on the surface of the ground. In Kansas (USA), rabbits sometimes gnaw exposed chalk (calcium carbonate). Rodents in particular need great quantities of calcium in their diet, because their front teeth grow continuously and are constantly being honed down with use. But this behavior is not unknown in humans, either. In areas where the diet would otherwise be deficient in calcium, as in one part of the Andes, people have the habit of grinding limestone and salting their food with it. When asked why they ingest this tasteless substance, they say it makes them feel better. To return to the snails: Why not let them decide what they want to gnaw? Give them a choice of several calcium-rich materials (say, a seashell, an eggshell, and a piece of limestone) and let them choose what suits them best. Then tell us all about it! Andrew K. Rindsberg Geological Survey of Alabama