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.
6. [TETRACLITA] VITIATA. Pl. [11], fig. [3 a]-[3 e].
Shell white, generally tinged in the upper part with pink; surface irregular: parietal tubes very irregular: radii moderately wide, with their summits oblique: alæ with very thick crenated sutural edges: tergum with the spur not joined to the basi-scutal angle; spur with its extremity equably rounded.
Hab.—Philippine Archipelago; Barrier Reef, (Raine’s Islet), Australia; Mus. Cuming and Stutchbury. Attached to massive corals, to coral-rock, to a Tridacna, and to [Tetraclita cœrulescens].
General Appearance.—Shell conical, moderately steep: white, generally with a tinge of pinkish-purple in the upper part, owing to the exposure of the tips of the upfilled parietal tubes. The lower part of the surface is generally well preserved, and is formed by very irregular, branching, longitudinal, slightly prominent, ridges. Radii of moderate width, with their summits oblique. Orifice rather large, rounded trigonal.