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. B. sulcatus. B. tintinnabulum. B. nigrescens. B. cylindraceus. B. calycularis. B. roseus. B. palmatus. B. stalactiferus. B. plicatus. B. duploconus. B. patellaris. B. semiplicatus. B. galeatus. B. ovularis. B. miser. B. amphimorphus. B. perforatus. B. lævis. B. spinosus. B. radiatus. B. subimbricatus. B. rugosus. B. placianus. B. crispatus. B. punctatus. B. fistulosus. B. latus. _B. sulcatus._ The furrowed Balanus. White; valves nearly smooth; operculum strongly ridged transversely, with longitudinal, nearly obsolete striæ. _B. tintinnabulum._ The little Bell Balanus. Pl. 4, fig. 1. Shell conical; valves strongly and irregularly ribbed; interstices delicately striated transversely; colour purple. _B. spinosus._ The spiny Balanus. Either has no support or a membranous one; armed with spines exteriorly. 4. Acasta. Three species. Found in sponge, from which, when detached, it cannot stand erect on account of the convexity of the base. Shell oval, sub-conic, shaped like a Patella, with six lateral unequal valves slightly connected; operculum with four valves. Acasta Montagui. A. glans. A. sulcata. _A. Montagui._ Montague’s Acasta. Valves erect, triangular, acute, with muricated ascending spines. 5. Creusia. Three species. The shells of this genus are generally small; found in the seas of hot countries attached to madrepore and other marine substances. Shell sessile, thin, Patella-shaped; aperture oval, rather large, closed by a large sub-pyramidal bivalve operculum; a considerable calcareous support, funnel-shaped, penetrating the bodies to which the animal is attached. Creusia stromia. C. spinulosa. C. verruca. _C. spinulosa._ The spiny Creusia.