Very depressed, circular, as if radiated by the disposition of marked spaces, striated transversely, forming six rays diverging from the centre to the circumference; aperture oval and hexagonal.
C. balænarum. The Whale Coronula.
A little more elevated; the spaces prominent, equal between them, much larger than the excavated; the aperture subcircular; the operculum of four valves, almost equal, occupying but a small space of the membranous part which forms between them a sort of tube.
C. diadema. The Crown Coronula.
More elevated, sub-hexagonal; the spaces almost equal, the hollow parts larger than the saliant; the superior aperture very large and hexagonal; the inferior much smaller, of the same form, and communicating in a round excavation by radiated plates; the operculum bivalve.
C. quinquevalvis. The five-valved Coronula. Pl. [4], fig. 3.
Species irregular, having only five valves, of a purple hue.
3. Balanus. The Acorn Shell. Twenty-eight species.
So named from its resemblance to an acorn. Shell conical; the coronary part formed of six very distinct valves, one dorsal, one ventral, and two pairs of laterals, with or without a calcareous support; operculum of four articulated pieces, forming a sort of pyramid in the superior aperture of the tube.
Balanus angulosus.