BIOLOGY How are seashells created? Or any other shell, such as a snail's or a turtle's? 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 wit