Column Barnacles

Adult shells are very small, rostrocarinal diameter of not more than 9 mm in solitary specimens, 4 mm in columnar growth individuals. Height of crowded individuals reaches to 16 mm, 6 mm in solitary specimens. The shell is whitish, and erodes to show pitting. Juvenile shell wall of 6 plates, becoming four plated by diameter of .5mm, and sutures fuse together, cannot be seen in adult, even when etched with acid. Basis is membraneous. Opercular plates are deeply interlocked with sinuous articulation, and subject to considerable environmental wear, making large variations in individual appearance and shape.471 Articulation is not nearly as deep as in Chamaesipho brunnea. Interior of scutum shows a wide shallow adductor pit, and small lateral depressor pit. There is a distinct low rounded scutal adductor ridge, in contrast to that of C. brunnea, which has at best poorly developed ridge. Scutal articular ridge, which in chthamaloids is large central lobe on tergal margin, is rounded. In C. brunnea, this lobe is very large and rectangular. The tergum is much narrower than in C. brunnea, with articular margin bearing two rounded relatively shallow re-entrants. Chamaesipho brunnea shows a single very deep re-entrant. Tergal depressor crests are prominent in both species, with fewer (to about four) in C. columna, versus up to seven in C. brunnea. …