Francis Horne, a biologist who studies shell formation at Texas State University, offers this answer.
The exoskeletons of snails and clams, or their shells in common parlance, differ from the endoskeletons of turtles in several ways. Seashells are the exoskeletons of mollusks such as snails, clams, oysters and many others. Such shells have three distinct layers and are composed mostly of calcium carbonate with only a small quantity of protein--no more than 2 percent. These shells, unlike typical animal structures, are not made up of cells. Mantle tissue that is located under and in contact with the shell secretes proteins and mineral extracellularly to form the shell. Think of laying down steel (protein) and pouring concrete (mineral) over it. Thus, seashells grow from the bottom up, or by adding material at the margins. Since their exoskeleton is not shed, molluscan shells must enlarge to accommodate body growth. This pattern of growth results in three distinct shell layers: an outer proteinaceous periosteum (uncalcified), a prismatic layer (calcified) and an inner pearly layer of nacre (calcified).
In comparison, turtle shells are part of the vertebrate animal's so-called endoskeleton, or skeleton from within the body. Surface scutes are epidermal structures, like our fingernails, made of the tough protein keratin. Underneath these scutes are the dermal tissue and calcified shell, or carapace, which is actually formed by fusion of vertebrae and ribs during development. By weight, such bone consists of about 33 percent protein and 66 percent hydroxyapatite, a mineral composed largely of calcium phosphate with only some calcium carbonate. Why exoskeletons of snails and clams are calcium carbonate while the endoskeleton of vertebrates like turtles are primarily calcium phosphate is not known. Both shells are strong, allow for protection, attachment of muscles and resist dissolution in water. Evolution works in mysterious ways.
Unlike seashells, turtle shells have living cells, blood vessels and nerves, including a large number of cells on the calcareous shell surface and scattered throughout its interior. Bone cells that cover the surface and are dispersed throughout the shell secrete protein and mineral and more or less entomb themselves. The bone can grow and reshape continuously. And when a bone breaks, cells are activated to repair the damage. In fact, turtle shell grows from within just like leg bones in humans. Nutrients such as protein and calcium are supplied by blood vessels within the bone, not from outside of the bone tissue. Damaged seashells, on the other hand, use secretions of proteins and calcium from the mantle cells underneath the shell for repair.
Construction of both turtle shells and seashells share some fundamental mechanical properties. The currently accepted understanding of how shell forms is that the protein matrix of bone and seashell is secreted out of the cells. These proteins tend to bind calcium ions while guiding and directing calcification. Binding of calcium ions to the protein matrix enhances crystal formation according to precise hierarchical arrangements. Exact details of this mechanism remain unclear in both turtles and seashells, but many proteins have been isolated that are known to play a role in shell formation. Whether the calcium carbonate crystal is calcite, as in the prismatic layer, or aragonite, as in the nacre of a seashell, seems to be protein-determined. Secretion of different kinds of proteins at different times and places in the seashells seems to direct the type of calcium carbonate crystal formed. Calcified bone or shell of turtles, on the other hand, does not readily form different crystals.
Whereas turtles grow their bones like humans or other land animals and thus make more room for themselves, snails and clams have to gradually enlarge and extend their shells by adding new organic matrix and mineral to the outer margins of the shell. The newest part of the snail shell, for example, is located around the opening where the animal pokes out. The outer edge of its mantle continuously adds new shell at this opening. First, an uncalcified layer of conchiolin--protein and chitin, a strengthening, naturally produced polymer--is formed. Then comes the highly calcified prismatic layer that is followed by the final pearly layer, or nacre. The iridescence of the nacre occurs, incidentally, because crystal aragonite platelets function like a diffraction grating in dispersing visible light. Sadly, turtles lack this mechanism, which keeps their shells more dull, but their shells are perfect for hiding in the undergrowth or murky waters. Clearly, not all shells are the same.