_Anomalies and exceptions._--In Tetraclita (Pl. 10, fig. 1 _g_, 1 _h_) from the branching of the longitudinal septa, several irregular rows of tubes are formed. In certain varieties of _Balanus balanoides_ (Pl. 7, fig. 2 _b_), and in _B. cariosus_ (Pl. 7, fig. 3 _b_), slight branching ridges on the internal surface of the walls, seem to answer to the longitudinal septa, and produce, during the downward growth of the shell, extremely irregular cells, and short tubes. In _Balanus vinaceus_ (Pl. 2, fig. 7 _d_), the internal lamina, instead of being solid, as in every other species, is left cancellated, and thus betrays, much more plainly than usual, its origin in the united denticuli of the adjoining longitudinal septa. In _Balanus porcatus_, between the main longitudinal septa, there are (Pl. 6, fig. 4 _e_) what may be called rudimentary and disconnected longitudinal septa. In Coronula and its allies (Pl. 16, fig. 6, and Pl. 17, fig. 4 _c_) it is the outer lamina of the compartment which is anomalous; for in the two or three lower zones of growth, it forms only a ledge on each side of the longitudinal septa; which ledges, higher up, become confluent, and so form an ordinary outer lamina. In Coronula, also, the wall of each compartment (see transverse section, Pl. 16, figs. 5, 7) is very remarkable from being deeply folded, the folds being on their internal faces firmly calcified together, and on their external faces closely pressed together (often with a neatly serrated suture), so that the whole nature of the shell might be, as has happened, easily quite misunderstood; and the walls be considered as very thick, instead of being, as is really the case, very thin. In Chelonobia (Pl. 15, fig. 1), however, the walls are truly of such great thickness, that the nature of the relative parts might likewise be misunderstood; in this genus the ovarian tubes enter the walls, extending up between the longitudinal septa, or, as they may here be more naturally called, the radiating septa. I will specify a few more peculiarities worthy of remark:--in some species of the sub-genus Acasta, clefts are left, covered only by membrane, on the lines of suture (Pl. 9, figs. 7 _a_, 8 _a_), between the compartments, just above the basis; and in other species the basis is perforated by numerous membrane-covered, minute orifices. In Platylepas, each compartment has one deep inward fold (Pl. 17, fig. 1), somewhat analogous to the three folds in Coronula; this fold is produced into an internal midrib, supporting and rendering convex the membranous basis; in this genus, also, the rostrum, owing to its midrib, is generally thrust a little on one side, and the shell thus rendered asymmetrical. In _Chamæsipho scutelliformis_ the shell is symmetrically perforated (Pl. 19, fig. 4 _a_) by four apertures. Lastly, in _Chthamalus Hembeli_ and _intertextus_, after a certain age, the basal edges of the walls become inflected, and continue to grow