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.
With respect to the rate of growth of this species, I am indebted to Mr. W. Thompson for the following note:—
“Sept. 29, 1848.—I examined a great number of Balani, in reference to the growth made by them during the present season, and found it to average three lines in diameter, and at most four lines. I saw a few minute specimens, only one line in diameter, showing that the species continued to breed until lately: these latter were probably not more than four weeks old. The young of the present year are plainly distinguished from the older ones, by their pure white colour and fresh appearance. Judging from the size of this year’s specimens, and of the older ones on the same stones, I am of opinion that the term of life of the species is two years. Of the older shells, which I examined and found living in the spring, nine tenths are now dead, the walls only remaining, the opercular valves having been washed away.”
Mr. Thompson goes on to say, that the individuals which had, on July 3, a basal diameter of from two and a half to three lines, had attained, by the 30th of September, a diameter of four and a half lines, this being here the maximum size of the species.
30. [BALANUS] CARIOSUS. Pl. [7], fig. [3 a]-[3 e].
LEPAS CARIOSA. Pallas. Nova Acta Acad. Scient. Petrop. tom. ii (1788), p. 240, Tab. 6, fig. 24.
Parietes thick, formed by several rows of unequal-sized pores. Tergum narrow, with the apex beaked, and spur sharply pointed.