Craniidae

Craniidae, a family of Brachiopoda, of which Crania is the type-genus. They are fixed by the flat ventral valve to rocks, echinoids, mollusca, etc. The upper or dorsal valve is limpet-shaped. The two valves are connected only by muscles. There is no hinge, so that the family belongs to the subclass Inarticulata. Crania was a common fossil in the Chalk. The family ranges from the Silurian upwards.