General Appearance.—Shell whitish, probably tinged, when alive, with reddish-purple; depressed; surface perfectly preserved, smooth, but having longitudinal very prominent ribs, almost invariably ten in number; namely, three on both the rostrum and carina, and two on the two lateral compartments, with ten corresponding projections round the basal margin. Orifice passing from rounded-trigonal to diamond-shaped. The radii are very broad and square at the summit, and extend from tip to tip of the compartments. Basal diameter of largest specimen under half an inch, generally from .3 to .4 of an inch.

Scuta, of the usual sub-triangular shape, and not transversely elongated, as in T. purpurascens. External surface striated longitudinally; in many specimens there is a medial depression, or a row of very small pits, such as occur on the scuta of Balanus trigonus and lævis. The adductor ridge is moderately developed, and runs nearly parallel to the occludent margin; there are no crests for the rostral and lateral depressor muscles.

Terga: these in area equal two thirds of the scuta: the spur is short and rounded, and placed as described under T. purpurascens; but the articular ridge seems to be more prominent than in that species.

Structure of the Shell and Radii.—The parietal tubes are small, and very numerous, as in T. purpurascens. The radii are wide, square on the summits, but not so conspicuously covered by hirsute epidermis as in that species. Internally, the tubes forming the radii are smaller, and run more transversely than in T. purpurascens, that is in the normal course, as in Balanus. The edges of the alæ are nearly or quite smooth. The Basis is as distinctly calcareous, as in the other species of the genus.

Mouth: the trophi are not so hairy as in T. purpurascens; the labrum seems destitute of teeth; the palpi are club-shaped at their ends; the mandibles have only three teeth. The second and third cirri are not so short and blunt relatively to the others as in T. purpurascens. In the posterior cirri, the elongated segments carry four main pairs of spines, between which there is no intermediate tuft of fine spines.

The Affinities of this species have been fully pointed out under the last and closely related species. In external appearance, T. costata can at first hardly be distinguished from those young and pale-coloured varieties of T. purpurascens, which have their external surface not corroded, and their radii well developed.