_Compartments._--The external appearance of the shell has already been described. In the most typical species, the parietes consist of an outer and inner lamina, separated by strong longitudinal septa; these septa are denticulated on both sides at their bases, but only close to the inner lamina; in fact the inner lamina is apparently formed by the union, thickening, and production, of some of the denticuli. As it is not the innermost of the denticuli on the basal edges of the longitudinal septa, which thus become united into a solid layer, the longitudinal septa form slightly projecting, longitudinal ribs on the inner lamina. These internal ribs are longitudinally striated; in old specimens they often become obliterated, especially in the upper part of the shell. The parietal tubes or pores (occupied by threads of corium) are generally square and large; but in _B. Ajax_ they are very small, and in _B. glandula_ often extremely minute. In the upper part of the shell, and sometimes low down, they are generally crossed by thin, transverse, calcareous septa: in some species, as in _B. perforatus_, and in some varieties of _B. amphitrite_, the upper ends of the tubes are filled up solidly with shell. In some varieties of _B. crenatus_ and of _amphitrite_, the longitudinal septa, near the outer lamina, divide, thus giving rise to a very imperfect row of outer short tubes. In _B. vinaceus_ (Pl. 2, fig. 7 _d_) the inner lamina is cancellated instead of being solid, which is caused by the basal denticuli of the longitudinal septa being simply united together by their ends and crossed by transverse septa, instead of being consolidated into a mass. In several species, as in _B. Hameri_, the walls consist only of the outer lamina with longitudinal ribs, no inner lamina having been formed; the ribs here evidently answer to the longitudinal septa in the foregoing species. In _B. flosculus_ and _imperator_ the walls are solid, their basal margins being formed of irregular, elongated points, and little ridges (Pl. 8, fig. 4 _c_), which apparently prefigure the more regular longitudinal ribs or septa. In _B. balanoides_ the walls are generally either nearly smooth and solid, or irregularly cancellated; in _B. cariosus_ (Pl. 7, fig. 3 _b_) two or three rows of short irregular tubes are formed by unequally branching septa, almost as in the genus _Tetraclita_.