The genus, as we have just seen, is hardly separated from the sub-genus [Acasta]; by [B. allium] it tends to pass through the [sub-genus Creusia] into [Pyrgoma]; and by [B. imperator] and [flosculus] it graduates into [Elminius] and [Tetraclita].
Geographical Distribution.—This, which is much the largest genus of sessile cirripedes, has its species scattered over the whole world, from the arctic regions, in lat. 74° 48′, where we have [B. crenatus] and [porcatus], throughout the tropical seas, to Cape Horn, where [B. flosculus] adheres to the coast-rocks. Many of the species have individually very wide ranges; thus [B. tintinnabulum] and [amphitrite] are found throughout the warmer seas; but the wide distribution of these species may be partly due to their frequent adhesion to ships’ bottoms: [B. crenatus] ranges from the frozen seas, in lat. 74° 48′ north, to the West Indies and Cape of Good Hope—a wonderful endurance of the most opposite climates. [Balanus improvisus], again, extends from Europe to Nova Scotia, thence southward to Patagonia, and up the western coast of S. America, someway north of the Equator. Most of the species have considerable ranges; thus of the six species found on the eastern shores of northern America, five of them occur in Great Britain. Of the thirty-six species of which the habitats are known, exactly one third, or twelve, inhabit both the torrid and temperate zones, these being divided by the isocryme of 68°; nine are found exclusively in the torrid, and fifteen exclusively in the temperate zones. Within the warmer latitudes, and especially in the southern hemisphere, [Tetraclita] and [Elminius] to a certain extent supplant [Balanus]. In depth, the species range from the upper limits of the tidal zone to even fifty fathoms. [Balanus improvisus] and [eburneus] are able to survive in brackish water. The different species are attached to various surfaces—rocks, shells, timber, floating objects, sea-weed, lamelliform corals, Milleporæ, Gorgoniæ, and even to sponges. Mr. G. B. Sowerby has remarked[85] that in the species from the southern hemisphere it is the basis, and in the species from the northern hemisphere it is the parietes, which are elongated, when the individuals, from being crowded together, become cylindrical; but this is erroneous; [B. perforatus], in the northern hemisphere, sometimes has an elongated basis; but no doubt the basis of our commonest species, as [B. balanoides], [crenatus], and [porcatus], from being either membranous or thin, does not become cup-shaped; whereas this structure is conspicuous in [B. psittacus] and [lævis], the two commonest species in southern South America.
[85] Darwin’s ‘Geology of S. America,’ p, 264.
Fossil Species.—Having already given, under the Family, some account of the geological history of sessile cirripedes, short as it is, I here only allude to the subject in order to state my conviction that species cannot be satisfactorily distinguished in a fossil state, and rarely in a recent state, without an examination of the opercular valves. Nothing, indeed, could have been easier than to have affixed names to many groups of specimens, having different aspects, but to feel sure that these were really distinct species requires better evidence than can be afforded by the shell, without the operculum. No doubt, in some of the smaller sections of the genus—for instance, in that characterised by a membranous basis—it would have been possible to have distinguished some or several fossil species; but such have not as yet been found. When the specimens are much fossilised, it is, indeed, difficult to make out the primary points of structure—namely, whether the parietes, radii, and basis are porose: to do this it is sometimes necessary to rub down, polish, and carefully examine, a transverse section of a piece of the shell.
Sections of the Genus.
Parietes, and basis, and radii permeated by pores.
Parietes and basis sometimes permeated by pores, sometimes not; radii not permeated by pores; shell elongated in its rostro-carinal axis; basis boat-shaped, attached to Gorgoniæ and Milleporæ.