Some burrowing bivalves have markedly different sculpture in juveniles and adults. Imagine yourself as a tiny juvenile clam: To you, sand grains are the size of bricks. So young burrowers commonly have relatively thick ridges to act as ratchets to move themselves against these "huge" sand grains. The adult clam doesn't need the sculpture; in fact, it makes it more difficult to glide through the sediment. Their shells are smooth... except at the umbo, where the juvenile stage of growth may still be preserved. I'd like to know more about why juvenile shells are shaped differently from adult shells of other kinds of mollusks. Andrew K. Rindsberg Geological Survey of Alabama