CIRR1PEDIA. 335 



The Cirripedia include three orders, of which only the 

 order TJwracica has ever been found in a fossil condition, or 

 is ever likely to be so. In this order are the common Acorn- 

 shells (Balanidce) and Barnacles (Lepadidce), in which the 

 body is protected by a more or less complete calcareous shell. 

 The Acorn-shells are generally known as the " Sessile Cirri- 

 pedes," because the shell is directly attached by its base 

 to some foreign body, whereas the Barnacles are commonly 

 known as the " Pedunculated Cirripedes," because the shell 

 is supported upon a stalk or " peduncle." Besides these, the 

 order Thoracica comprises a third family, that of the Ver- 

 rucidce, in which the shell resembles that of the Balanidce in 

 being sessile, but differs in being unsymmetrical, and in some 

 other particulars. 



It is with the shell of the Cirripedes that the palaeontologist 

 has to deal ; and we may therefore consider briefly the chief 

 parts of the shell in the Sessile and Pedunculated Cirripedes 

 respectively. It will not be necessary, however, to enter into 

 minute details on this complicated subject, and it will be 

 sufficient to indicate the leading facts of importance. 



In the symmetrical Sessile Cirripedes or Balanidce, com- 

 monly known as Acorn-shells, the animal is protected by a 

 calcareous shell formed by calcifications within the walls of 

 the first three cephalic segments. The animal is placed 

 within the shell, head downwards, and is fixed to the centre 

 of a shelly or membranous plate, which closes the lower 

 aperture of the shell, and which is termed the " basis " (fig. 

 197, A, /). The " basis " is fixed by its outer surface to some 

 foreign object, and is sometimes compact, sometimes porous. 

 Above the basis rises a limpet-shaped, conical, or cylindrical 

 shell, which is open at the top, but is capable of being com- 

 pletely closed by a pyramidal lid or " operculum." Leaving 

 the operculum out of consideration at present, the sides of 

 the shell are seen to be composed of from four to eight 

 separate pieces or valves, or, as they are technically called, 

 compartments. These compartments are usually closely con- 

 tiguous by their lateral margins, and are separated by lines 

 of division or " sutures ;" but they are sometimes anchylosed 

 together. Each compartment consists of a central portion, 



