Tretenterata, a subclass of Brachiopoda or Lamp Shells, comprising those forms in which the valves of the shell are not hinged but held together by muscles, and in which there is an anus. The subclass comprises five families, of which two are extinct, while living representatives of the others are very rare. The best known genus is Lingula, the type of the family Lingulidae, which ranges from the Cambrian to the present with extremely slight variations. Crania, which lives attached to rocks, mollusca, etc., and ranges from the Ordovician (Lower Silurian); the shell in this is small, and the free valve is somewhat pyramidal; it is the type of the Craniidae. The other three families are the Obolidae, Trimerellidae, and Discinidae.