Seashells have three distinct layers and are composed mostly of calcium carbonate with only a small quantity of protein--no more than 2 percent. Mantle tissue that is located under and in contact with the shell secretes proteins and mineral extracellularly to form the shell. Seashells grow from the bottom up, or by adding material at the margins. 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.