Who among us hasn’t marveled at the diversity and beauty of shells? Or picked one up, held it to our ear, and then gazed in wonder at its shape and hue? Many a lifelong shell collector has cut teeth (and toes) on the beaches of the Jersey Shore, the Outer Banks, or the coasts of Sanibel Island. Some have even dived to the depths of the ocean. But most of us are not familiar with the biological origin of shells, their role in explaining evolutionary history, and the incredible variety of forms in which they come. Shells are the external skeletons of mollusks, an ancient and diverse phylum of invertebrates that are in the earliest fossil record of multicellular life over 500 million years ago. There are over 100,000 kinds of recorded mollusks, and some estimate that there are over a million more that have yet to be discovered. Some breathe air, others live in fresh water, but most live in the ocean. They range in size from a grain of sand to a beach ball and in weight from a...