General Appearance.—Shell conical, with the orifice oval, unusually small, being generally only from one third to half of the basal diameter; sometimes moderately large; in one single instance as wide as the basis. Radii, often represented by mere lineal fissures, or they are narrow, or sometimes moderately wide. Colour pale, dull purple, sometimes lilac, often passing into a dead pure white: the same individual will occasionally have one part of its shell white, and another purple: the purple tint almost invariably is nearly uniform, or not in stripes. The radii are generally white, when the whole shell is purple, but sometimes they are pale purple: the sheath is apparently always coloured of a fine claret-purple, with the triangular portion of the alæ, added during diametric growth, generally white, but sometimes purple. The surface is quite smooth, but very often, especially on the shores of England, whole groups of specimens (excepting the very young ones,) have had the outer lamina of the parietes entirely corroded and removed; in this case the shell assumes a dirty, more or less dark, ash-colour, feebly tinted with purple, and the whole surface, owing to the exposure of the solidly filled-up parietal tubes, becomes finely striated, or covered with very narrow, longitudinal ridges. When specimens are crowded together they often become cylindrical, and much elongated, owing to the basis becoming deeply cup-formed: I have seen specimens, half an inch in diameter in the widest part, one inch and a half in height, the walls forming only a third of this. The largest specimen which I have seen (from the southern shores of England) had a basal diameter of 1.2 of an inch; some very steeply conical specimens were .9 of an inch in height, and .8 in basal diameter.

Scuta, externally, slightly convex; growth-ridges approximate, moderately prominent. Internally (Pl. 4, fig. 3 a) the articular ridge is moderately developed, with the lower end produced downwards into a freely depending, flattened style, somewhat variable in size, but not so long as in B. lævis, and easily broken in disarticulating the valves. The adductor ridge is very prominent, running from almost the apex of the valve, close to the articular ridge, to near the basal margin. The basi-tergal portion of the valve is converted by the adductor ridge into a rather deep cavity, within which there is a short, sharp, and minute ridge, close and parallel to the adductor ridge, and bounding the impression left by the lateral depressor muscle: this insignificant ridge was present in every specimen; it occurs only in very few other species, as in B. nubilus and cariosus. The thickness of the valve sometimes varies a little, and when thick the adductor ridge does not appear quite so prominent. Tergum, with the apex moderately beaked and produced; beak triangular in section, coloured dark purple, as is the upper internal surface of the valve; the longitudinal furrow is deep, and has its edges folded in, and even quite closed. The spur is moderately long and narrow; but its width varies a little (Pl. 4, fig. 3 b, 3 c), and consequently it stands at either rather above or at twice its own breadth from the basi-scutal angle: its lower end is either bluntly pointed or square, and generally is feebly toothed on the under-side. The basal margin of the valve generally slopes a little, on both sides, towards the spur. Internally, the scutal margin is but slightly inflected: the articular ridge is but slightly prominent, and but little curved; in the upper part of the valve there are generally several very minute ridges, parallel to the articular ridge, on the side towards the scutum. The internal surface of the spur itself is sometimes concave. The crests for the carinal depressor muscle are barely developed. It may here be mentioned that on the opercular membrane many long spines stand rudely arranged in rows.

Parietes: the parietal tubes have not transverse septa; but are solidly filled up in their upper parts by dark-purple layers of shell. The radii, as already stated, are either not at all developed, or are extremely or only moderately narrow, with their summits more or less oblique: in Mr. Cuming’s collection, however, there is an unique specimen, var. mirabilis (Pl. 5, fig. 1 c) with the aperture of the shell as wide as the basis, with bright purple parietes, and white, very broad radii, having their summits parallel to the basis. The septa of the radii are finely denticulated, and the interspaces are filled up solidly. The alæ have very oblique summits, and their edges are finely crenated. Basis, flat, or deeply cup-shaped; there is often an underlying, coarsely-cancellated layer.

Mouth: labrum finely hairy, but without any teeth; mandibles, with the 4th tooth small; the 5th confluent, with the sometimes smooth, sometimes pectinated inferior angle. Maxillæ, rather broad, with a slight notch under the upper pair of spines. Cirri, first pair, with one ramus, having 29 segments, and above one third longer than the shorter ramus, having 17 segments; these latter segments are remarkable by the extent to which their upper front surfaces are laterally produced into projections, twice as long as the breadth of that portion of the segment which is articulated to the adjoining segment. These projections have a double row of serrated spines on their upper edge, and a beautiful radiating bundle at the end; the projections decrease in length, both in the upper and lower segments. The second cirrus (Pl. 29, fig. 4) has the segments (13 in number, in the same individual with the segments above enumerated) of both rami produced in the same singular manner as in the first pair. The third pair have only inverted conical segments, coloured darker purple than the other cirri. The sixth pair had in the same individual 31 or 32 segments, and therefore one or two more than in the longer ramus of the first pair.[93] The segments in the posterior cirri have their anterior faces shield-shaped, and bear 6 or 7 pairs of spines, with some minute intermediate spines. There is the usual point at the dorsal basis of the penis.

Range.—This species is common on the southern shores of England and in the Channel Islands: the largest specimens which I have seen came from these quarters. The most northern point whence I have seen specimens, is Tenby, in South Wales. This species is common throughout the Mediterranean; I have seen specimens from Malaga, Sicily, Algiers, and Smyrna; thence it ranges down the western coast of Africa, as far south as the Gambia and Loanda, in 9° south latitude. I believe British specimens are more often corroded than those from further south. Amongst some old, ill-kept specimens in a box in the British Museum, marked “Kingston, Jamaica,” there were some of this species: also I received some specimens, marked “S. America,” from Mr. G. B. Sowerby: again, Ellis, in Phil. Trans., vol. 50, part 11, gives a figure (Tab. 34, fig. 15) of some specimens from the West Indies, which I believe to be B. perforatus: hence, it is in some degree probable that this species, like B. tintinnabulum, and amphitrite, and improvisus, may be found on both sides of the equatorial Atlantic. Balanus perforatus is attached, together with B. tulipiformis, trigonus, amphitrite, Chthamalus stellatus, and Pollicipes cornucopia, usually to rocks, near the lower limit of the tidal level; but I believe, from specimens kindly sent me by Mr. Mac Andrew, that it is frequently obtained by dredging; one specimen was even marked 30 fathoms. According to Poli, it is sometimes attached to the bottoms of vessels; and I have seen a specimen adhering to the floating Lepas Hillii.

Affinities.—This is a distinct species, closely allied to no other species, but comes nearest to B. lævis, which is its representative in Southern America, and on the whole west coast of that continent. It is allied to that species, and differs from most other species, in the general form of the shell, its small orifice, narrow radii, and often deeply cup-formed basis. It agrees to a certain extent in the colouring, though the purple here is much more prevalent, and is not confined to the shelly matter filling up the parietal tubes. It agrees with that species in the general structure of the scutum; but the two or three deep, longitudinal furrows are here absent; and the minute ridge, parallel to and almost under the adductor ridge, is a peculiarity confined to this and very few species in the genus. The terga differ from those of B. lævis, chiefly in the spur being narrower, and in the apex being beaked. Lastly, the highly protuberant segments of the one ramus in the first cirrus, and of both rami in the second pair, are here remarkable. With regard to the varieties, I have nothing to add to their short diagnostic characters above given.