General Appearance.—The shell, in middle-sized and old specimens, is almost invariably folded longitudinally and irregularly; it is either dirty white or very often pale brown, and punctured from the outer lamina having been corroded, to which action it is extremely subject. In very young specimens, the surface is usually quite white and smooth. The shell is sometimes much depressed; generally conical, but when crowded together, cylindrical or club-shaped, one specimen being even more than five-and-a-half times as long as wide. In Mr. Jeffreys’ collection there is a specimen 2.5 of an inch long, .45 in diameter at the summit, only .2 in the middle, and rather more than .2 near the base. Another specimen was 1.8 in length, its greatest diameter being .35 of an inch at the summit. On the other hand, I have seen a very depressed variety, with deeply folded walls, in Mr. Thompson’s collection from near Dublin, which was no less than four times as wide as high; so that the difference in proportion of height and greatest width, in the two extreme specimens, was nearly as 10 to 1. Occasionally, from some unknown cause, isolated specimens become cylindrical. The orifice of the shell, in the much elongated specimens, is generally deeply toothed. The radii are always narrow, sometimes extremely narrow, and have their summits smooth and rounded.

English specimens do not usually attain half an inch in basal diameter; I have, however, seen one from near Yarmouth .9 of an inch in diameter. Specimens from Massachussetts seem rather larger than the average size of British specimens, many being .6 of an inch, and one specimen a whole inch in basal diameter.

The opercular valves so closely resemble those of B. crenatus, that the description is necessarily comparative; in some cases they could hardly be discriminated; generally, owing to the disintegration to which this species is subject, the tips of the scuta are worn off, and hence the articular ridges together form (Pl. 7, fig. 2 a) a square projection, indenting the two terga; but I have examined young specimens and others when not disintegrated, in which the opercular valves, viewed externally, presented no difference whatever from those of B. crenatus. The scuta, however, are, I think, generally rather thicker, with the growth-ridges more prominent, and with the tips certainly less reflexed than is usual with B. crenatus. Internally, the articular ridge is rather less prominent: there is no distinct adductor ridge. The terga are often rather narrower in proportion, and this especially holds good in the elongated varieties; in these latter, there is occasionally a moderately deep longitudinal furrow: the spur is often exactly the same shape as in B. crenatus, but it is apt to be rather longer (Pl. 7, fig. 2 c) and more pointed: in var. (a) it is pointed (fig. 2 d) in a very remarkable manner. Internally, the articular ridge is decidedly more prominent than in B. crenatus; the crests for the tergal depressor muscles are either well developed or almost absent. From this description it will be seen, how singularly the opercular valves of the common varieties of these two species resemble each other. I may mention that in some of the much elongated specimens, the muscles going to the opercular valves partially lose their transverse striæ, and become ligamentous.

The Parietes are either quite solid, or more commonly are permeated by minute pores, or by small irregular square tubes (Pl. 7, fig. 2 b), which only run up each successive zone of growth, for very short distances, giving to the shell a cancellated structure, which from corrosion is often externally visible. In the rather rare variety (a) the parietes are permeated by regular tubes, extending up to the apices of the compartments, but crossed by transverse septa. The longitudinal septa, when such can be said to occur, in no case are denticulated at their bases. The internal surface of the parietes is either quite smooth or is traversed (Pl. 7, fig. 2 b) by very slight anastomosing ridges, but never, even in var. (a), by regular longitudinal ribs, as in most other species. The carinal margin of each compartment, on the inside, projects, as in B. crenatus, inwards, beyond the general surface of the shell, and running down, rests on the basal membrane. The lower edge of the sheath is rarely hollow beneath. The walls are lined by purplish, or pale brown, or sometimes by almost black corium; numerous tubuli penetrate the under sides of the walls and opercular valves; and it is the intersection of these tubuli that gives the punctured appearance to the often corroded surface of the shell. The radii are narrow, generally very narrow; they have their upper and outer margins, as seen externally, very oblique, rounded and (when well preserved) smooth; their sutural edges are either quite smooth, or sometimes just perceptibly pitted, like the basal margin of the walls, or occasionally furnished with globular or arborescent little ridges. The alæ are also very oblique, but to a variable degree, sometimes only slightly oblique: their sutural edges are either smooth or obscurely crenated. Basis, membranous; in some much elongated specimens, during continued growth, the basal edges of the compartments approach each other so closely as almost to touch, so that the whole shell becomes pointed at the bottom; but on careful inspection I have never failed to find, even in the most pointed specimens, a minute basal membrane; in other much elongated varieties, in which the shell has apparently become too large for the animal’s body, the basal membrane, instead of being flat, becomes drawn up deeply inwards, so as to touch the surface of attachment only close round the basal edges of the shell.

Mouth: labrum with the teeth on each side of the central notch unusually variable in number; I have seen specimens with only two on each side, with four on each side, with five on one side and four on the other, with five on one side and none on the other, and with six on both sides; hence the total number ranges from four to twelve. Mandibles, with the fourth and fifth teeth small, or quite rudimentary. Maxillæ, with scarcely even a trace of a notch under the upper pair of spines. Cirri; first pair, with one ramus one third or one fourth longer than the other; in one specimen the number of segments were nine and sixteen in the two rami: second and third cirri short, very nearly equal in length, having in the just-mentioned specimen respectively ten and eleven segments; the sixth cirrus in this same specimen had twenty-five segments, each segment being about as long as broad, and supporting six pairs of spines. In the singular variety (a) the posterior cirri are more elongated, and each segment supports seven or eight, and in one case even ten pairs of spines! the third pair is also in this variety proportionally rather longer. At the base of the third pair there is a tuft of fine spines. The penis has not, as in B. crenatus, a point at its dorsal basis. The branchiæ are very little plicated.

Of the varieties having much elongated, club-shaped, hour-glass shaped, and depressed shells, there is no necessity to say anything in particular. With respect to the remarkable variety (a), I at first named and described it as a distinct species: I have received two lots, both from North America, one being sent me by Professor Agassiz from Cape Cod. These agreed in having the parietes permeated by regular tubes; in having the spur of the tergum most sharply pointed; in the third pair of cirri being proportionally longer compared with the second pair; in the sixth pair having more numerous segments, namely, three times as many as in the third pair; in the segments of the posterior cirri being more elongated, and especially in the number of pairs of spines on each segment—amounting in one case even to ten, a number unparalleled in other cirripedes. It may naturally be asked why I have not retained so well marked a form as a distinct species? In the first place, I found the most remarkable character in var. (a), namely, the number of pairs of spines on the posterior cirri variable, there being in one lot seven or eight pairs, and in the other lot nine or ten pairs on each segment. Secondly, all the characters by which this variety differs from the common B. balanoides, are those which are variable in the latter; this is especially the case with the structure of the parietes, and in a lesser degree with the spur of the tergum. Thirdly, I found a specimen in Mr. Cuming’s collection, from Sweden (so that this var. (a) is not confined to North America), in which the cirri quite resembled those of the American specimens, but the spur of the tergum was in an intermediate condition as compared to that of ordinary varieties; and the parietal tubes were of unequal sizes, and scarcely more regular than sometimes in the true B. balanoides. And lastly, I have seen specimens from Ayrshire, with the parietes permeated by regular tubes, but with the tergum in an intermediate condition, and with the segments of the posterior cirri not more numerous or more elongated than in B. balanoides, supporting only six or seven pairs of spines, that is only one more than is common with B. balanoides; so that it was impossible to decide whether to rank the Ayrshire specimen under var. (a) or under the common form, so that I was compelled to give up var. (a) as a species.

Monstrous individuals, with the male organs aborted: Parasite.—Amongst some specimens, chiefly elongated ones, sent to me from Tenby, in South Wales, I found no less than seven individuals with some of the posterior cirri distorted, unequal on the opposite sides, and in an almost rudimentary condition, and in each case with the penis truncated, without any muscle entering the stump, which was absolutely imperforate: the vesiculæ seminales were much shrunk; in one case without any zoosperms; in another case with headless zoosperms cohering in an unusual manner; hence it is certain that these individuals were functionally only female, and could not impregnate their own ova; yet in two instances the ova had been impregnated, no doubt by neighbouring perfect individuals, for they contained well-developed larvæ. Several of these monstrous individuals were infested by one, two, or three curious crustaceans, which have been described by Mr. Goodsir,[97] as the male of the Balanus; but these supposed males are females, and were distended with ova containing almost mature larvæ; I believe that they are the females of the unnamed genus, belonging to the family of Ioniens, described by Mr. Goodsir, which live parasitic within the sack (as I likewise found) of the same individual Balani.

Diagnosis.—I have seen several specimens of this species and of B. crenatus, absolutely undistinguishable in external appearance. I may specify one of B. balanoides, imbedded in an alcyonidium, and one of B. crenatus, imbedded in a sponge, and therefore neither at all abraded. Generally, the tips of the scuta in B. crenatus are a little reflexed, whereas in B. balanoides, when the shell has been at all disintegrated, the tips form a square projection locked into the terga. Bal. crenatus never assumes the punctured appearance so common in B. balanoides. Very young specimens of the latter can be distinguished by their dead white colour and smoothness. The edges of the radii are almost always smoother than in B. crenatus, and they are never so wide as is sometimes the case with B. crenatus. When a specimen is disarticulated, our present species can at once be distinguished from B. crenatus (and from B. improvisus), by its membranous basis, and by the solid or cancellated walls, which are rarely permeated by regular tubes or pores; and the walls when porose are not internally ribbed. I have already pointed out the few very trifling points, in which the opercula of the two species differ. The mouth and cirri offer likewise very few differences: in B. balanoides there are often more teeth on the labrum than in B. crenatus; the rami of the first cirri are perhaps here rather less unequal; the second and third pairs of cirri are certainly in most cases more equal in length; and lastly, the segments of the sixth cirri, even in the common varieties, bear, in equal-sized specimens, more pairs of spines than in B. crenatus. We shall see that in habits, with regard to depth, the two species differ, B. balanoides inhabiting much shallower water than B. crenatus.

Range, Habitats, &c.—This species is extraordinarily abundant within the tidal limits round the shores of Great Britain, and apparently of the northern United States. Besides numerous specimens sent to me from very many English localities, the late Mr. W. Thompson, of Belfast, kindly placed in my hands his very large collection; from these materials it appears that B. balanoides is the only tidal species in the northern parts of our island; but in the south and south-west, it is associated with the Chthamalus stellatus and Balanus perforatus. I doubt whether this species ever lives below the lowest tides; the case of a few specimens being mingled with B. improvisus and crenatus, (mentioned under the latter species,) at the bottom of a rudder of a small vessel, about six feet deep, is hardly an exception, for the water would there be troubled and aërated almost as in a breaker; and on this very rudder the upper two or three feet were coated exclusively by the B. balanoides. This species lives on rocks at both the uppermost and lowest limit of the tides; I am informed by Mr. Thompson, that he has seen specimens attached to a spot not covered by water during neap-tides. As a proof of its tenacity of life, Mr. Thompson informs me that he accidentally kept some specimens in a box, in a warm sitting-room, and found them alive seven days afterwards. This same most accurate observer finds, however, that B. balanoides is very susceptible to brackish water; he says, “that having kept some specimens alive for a week in excellent health, the water being changed once in thirty-six hours, they were one day killed instantly by some water, though brought from the same part of the estuary as usual, having been rendered brackish by much rain having lately fallen.” I may recall the fact, that B. improvisus lives daily for hours in absolutely fresh running water.

The B. balanoides lives attached, often continuously coating many square feet of the surface, to rocks, pebbles, wooden-piers, littoral shells and ulvæ. The most northern point whence I have received specimens, is in lat. 66° 34′ in North America, collected by Mr. Sutherland; and the most southern point is Delaware Bay, in the United States, in lat. 39°; I do not believe that this species extends into the Mediterranean, for Ranzani (Mem. di Storia Nat.), who particularly attended to the nature of the basis, was not acquainted with any Balanus having a membranous basis; and Poli (Test. Ut. Siciliæ,) describes only two species thus characterised, and these are manifestly Chthamali.