Scuta: these are longitudinally and finely striated; the basi-tergal corner is more rounded off than in [B. allium], and the articular ridge is not nearly so prominent: internally, the adductor ridge is rather more prominent. The Tergum is rather broader: its apex is produced into a minute sharp point: the scutal margin is straight; the spur is broader, and measured from the basi-scutal angle of the valve, considerably longer; namely, as long as half the width of the basal margin of the spur, whereas in [B. allium] it is only about a quarter as long as the basal margin of the spur: the lower edge of the spur is not here so directly transverse to the longitudinal axis of the valve as in [B. allium]: the external surface is not so flat as in that species, and a depression runs down to the basi-scutal angle of the spur.
Considering the difference in the shape and appearance of the shell, with its narrow radii and small orifice; considering the less strongly ribbed internal lamina of the parietes, the finer septa on the sutural edges of the radii, the slight difference in outline in the scuta and terga, more especially the greater length of the spur, I conceive I am right in ranking this form as a distinct species, though assuredly it is very closely allied to [B. allium], and even still closer to the following [B. quadrivittatus].
36. [BALANUS] QUADRIVITTATUS. Pl. [8], fig. [1].
Shell steeply conical, having four longitudinal gray bands placed crosswise: radii with their summits oblique: basis thin, solid. Scutum, with the lines of growth smooth; no distinct pit for the lateral depressor muscle: tergum as in [B. cepa].
Hab.—East Indian Archipelago, attached to lamelliferous corals, and associated with [Pyrgoma grande] and [Creusia spinulosa], Mus. Brit. and Stutchbury and Darwin. Philippine Archipelago, attached to a [Tetraclita], Mus. Cuming.
I have seen four sets of specimens of this species, taken in four different places, one set containing above twenty individuals, and all resembled each other exactly: nevertheless, this species comes so close to [B. cepa], that I am somewhat doubtful about its specific distinctness.
General Appearance.—Shell smooth, or slightly folded, steeply conical; white, with four longitudinal bands of pale brownish-gray colour, namely, on the rostrum, the carina, and the two lateral compartments: the carino-lateral compartments are very narrow and almost white: the four brownish-gray bands are darkest in the upper part of the shell, though always rather faint, and die out towards the base: they can sometimes be seen to be formed of several narrow longitudinal stripes; the tint shows a trace of containing purple. The orifice of the shell is small, rhomboidal, and not quite entire, owing to the obliquity of the summits of the moderately broad radii. In structure, the shell, radii, and alæ resemble those in the last species. The basis, however, does not appear to be permeated by pores. Basal diameter of largest specimen .25 of an inch.
The Scuta most closely resemble those of [B. cepa], but the lines of growth are not crenated, and internally there is only a very minute pit for the lateral depressor muscle, placed almost on the edge itself of the valve. The Tergum hardly differs at all from that of [B. cepa], but is perhaps of rather greater breadth.
The Mouth does not differ from that in the last two species. In the Cirri, the three posterior pairs have elongated segments, bearing only three pairs of spines, of which the lowest pair is minute: in [B. allium], and I believe in [B. cepa], there are five pairs of spines on each segment.