A class in the phylum Mollusca, comprising two orders, Gadilida and Dentaliida. The class is wholly marine; it is common and probably most diverse in the deep sea, and poorly represented in brackish, littoral, or estuarine habitats. Scaphopods are characterized by a tapering tubular shell with two apertures: the large ventral aperture that the foot extends out of and a small dorsal aperture. The tapering shells found in the Dentaliida give the class its common names tusk or tooth shells. Adult scaphopods range in length from about 3 to 150 mm. The shell consists of aragonite arranged in three or four layers.