a, the fixed valve ; b, the calcareous tube.
Clavagella, or CLUB-SHELL, a genus of marine Lamellibranchiate Molluscs of the same family with Aspergillum (q.v.). They mostly live in excavated holes in rocks or in masses of coral. The ordinary form of the bivalve shell is curiously modified. One valve (the right) is fixed to the inner surface of the chamber in which the animal lives, and is continued without interruption into a secondary shell-tube, which extends from the chamber outwards, and varies considerably in length in different species. The left valve is free and movable on the right. The mode of excavation is not known. Six living species of wide distribution are known. The fossil forms, which are more numerous, were first discovered. They do not occur below the Upper Chalk.