The tusk shells are a separate group from the snails, clams and other mollusks, and are known from all world oceans. They are thought to be the last molluscan class to evolve, though much remains to be learned about their derivation. Fully marine, they live partly buried in mud and sand on the sea bed. Historically, their distinctive shells were used as money and decoration by societies in different parts of the world. The bright green Elephant Tusk is found from the Red Sea to the Philippines, and is relatively common.