Thoracica, one of the four orders of Cirripedia, including those in which the body is protected by a series of calcareous plates. It is the only order that has known fossil representatives. It includes three families:- The Acorn-shells, or Balanidae, in which the shell consists of from four to eight valves or compartments; the oldest known form is the Protobalanus of the Devonian rocks of North America, while Balanus, the genus so common on rocks between tide-lines on the British coast, dates from the Cretaceous. The second family, or Verrucidae, is less important; the shell somewhat resembles that of the Acorn-shells. but it has only six valves. The type-genus Verruca is known in the Chalk, and still lives. The third family, or Lepadidae, includes the Barnacles; it is the best known living family, and dates back the farthest, as Plumulites (or Turrilepas), a fossil from the Wenlock Limestone of Dudley and beds of the same age in Bohemia, is referred to it. The living genus Scalpellum occurs also in the Chalk, while Pollicipes ranges from the Rhaetic period to the present.