[4/5]

Cirripedia without a peduncle; scuta and terga furnished with depressor muscles; other valves united immovably together. Scutum and tergum artculated together, subtriangular; compartments six; basis calcareous or membranous. The genus Balanus already includes 45 species, recent and fossil, and consequently the author divides the genus into sections, on characters derived from the porosity of the parietes, radii, and basis; and on whether the basis is membranous or calcareous. The genus is quite distinct from all the other genera of sessile cirripedes, with the exception of the subgenus Acasta, from which its separation, it must be confessed, is in one sense artificial; for the species of this subgenus graduate into those of Balanus, which have their shells elongated in the rostrocarinal axis, and which live attached to Gorgoniae. Pyrgoma genus can at once be recognized by the shell consisting of a single piece without sutures, whether viewed externally or internally, and by the cup-shaped basis, attached and often embedded in corals.