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.