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The word "seashells" is often used to mean only the shells of marine mollusks. Marine mollusk shells that are familiar to beachcombers and thus most likely to be called "seashells" are the shells of marine species of bivalves (or clams), gastropods (or snails), scaphopods (or tusk shells), polyplacophorans (or chitons), and cephalopods (such as nautilus and spirula). The tusk shells or scaphopods are a class of marine mollusks which vary in size from very small to medium sized. The scientific name of this class is Scaphopoda, meaning "shovel-footed". The shells of scaphopods are conical and curved in a planispiral way, and they are usually whitish in color. Because of these characters, the shell somewhat resembles a miniature elephant's tusk, hence the common name tusk shell. However, unlike an elephant's tusk, the shells of these mollusks are hollow and open at both ends
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