_General appearance._--The shape of the shell in the different species varies from depressed conical to cylindrical; the latter form being generally assumed when specimens are crowded together; but some species, as _B. balanoides_, _crenatus_, and _lævis_, seem more subject than others to be thus affected. The colour is either white, generally tinted by the yellowish or brownish epidermis, or any colour intermediate between bright pink and rich blue, purple being the prevailing tint. The persistence of the so-called epidermis is very different in different species, being even sometimes highly variable in the same species. The surface is either smooth or more commonly folded longitudinally, or sharply ribbed. The orifice differs in form from diamond-shape to trigonal; the carinal end, owing to the shape of the carina, being always sharper or narrower than the rostral end. The size of the orifice, in proportion to the shell, varies accordingly as the latter is more or less conical or cylindrical. The orifice is either entire or more or less deeply toothed, in proportion to the degree of obliquity of the summits of the radii and alæ. The radii almost always have smoother surfaces than the parietes. In some few species the radii are not developed, the sutures being marked only by fissure-like lines; in others they are very narrow, and in this case their upper margins are generally rounded and smooth, instead of being straight and jagged. The carino-lateral compartments are usually much narrower than the lateral compartments, occasionally in an extreme degree, as in _B. allium_. The shell is generally strong, sometimes to a wonderful degree; but the strength and thickness vary in the individuals of some of the species. By the action of hot caustic potash, the compartments in several species, such as _B. Hameri_ and _crenatus_, separate on a touch; in others, they adhere so strongly as to prove that the sutures must be calcified together. In this genus we have the largest known sessile cirripede, viz., the _B. psittacus_, and on the other hand many small species; but it is very difficult, except in well-known species, to ascertain the average or even the maximum dimensions.