About 310 million years ago, there were many varieties of shelled animals, but their shells were not as necessary for defense as in later ages. Gastropods, including drilling univalves, periwinkles, and snails, were present in the ancient seas. Bivalve gastropods, such as mussels, clams, oysters, and scallops, have remained practically unchanged through the millions of years since that time. Shelled and valved organisms also evolved, with brachiopods living in these ancient seas in much the same form as today, equipped with hinges, teeth, and other protective devices. [1]