Tusk shell

The tusk shells or tooth shells, technically the Scaphopoda /skæˈfɒpədə/ (the scaphopods /ˈskæfəpɒdz/, from Ancient Greek σκᾰ́φης skáphē "boat" and πούς poús "foot"), are members of a class of shelled marine mollusc with worldwide distribution, and are the only class of exclusively infaunal marine molluscs. Shells of species within this class range from about 0.5 to 15 cm in length. Members of the order Dentaliida tend to be significantly larger than those of the order Gadilida.

Tusk shells
Temporal range: Mississippian–Recent[1][2]
Various Scaphopoda, from left to right: Fissidentalium, Gadilida, Gadila, and Gadilida.
Scientific classification
Kingdom: Animalia
Phylum: Mollusca
Subphylum: Conchifera
Class: Scaphopoda
Bronn, 1862
Orders
Dentalium octangulatum
Fossil of Entalis laevis.
Tusk shell necklace from Bronze Age (MHNT).

These molluscs live in soft substrates offshore (usually not intertidally). Because of this subtidal habitat and the small size of most species, many beachcombers are unfamiliar with them; their shells are not as common or as easily visible in the beach drift as the shells of sea snails and clams.

Molecular data suggest that the scaphopods are a sister group to the cephalopods, although higher-level molluscan phylogeny remains somewhat unresolved.[3]