The _Parietes_, though moderately thick, yet are light and fragile; the denticuli at the bases of the longitudinal septa are prominent, and those on the adjoining septa are united together, making a network (Pl. 2, fig. 7 _d_), but the interspaces between them are not filled up by solid calcareous matter (as is the case with every other species of the genus), but are only crossed at successive levels by fine transverse calcareous septa; the internal lamina thus becoming cancellated, and, though thick, fragile. Hence, in a transverse section of the parietes, the ordinary parietal tubes or pores are seen to be lined on their inner sides by five or six rows of very minute pores. I have not seen any other instance of this structure. The internal lamina is ribbed, as usual, on its inner surface, by the projection of the longitudinal septa. The ordinary parietal tubes are open, to nearly the summit of the shell. The _radii_ are rather thin, and unusually fragile; their summits are parallel to the basis: their septa, as seen on the sutural edges, are extremely thin and denticulated on both their upper and lower surfaces, on the side towards the internal lamina: towards the external lamina, the septa are simple, and the small square pores thus formed, are open or not filled up. The _alæ_ have their summits extremely oblique, being added to very little during the diametric growth of the shell; the narrow margin, however, which is thus added, is coloured red, the rest of the sheath being nearly colourless: the sutural edges of the alæ are smooth. The _basis_ has a thick, underlying, finely cancellated layer of shell. _Animal's body_ unknown. A young specimen, .2 of an inch in basal diameter, differed from the above in being of a much paler purplish-brown. This species is distinct from all its congeners, in its peculiar colour, and likewise in the structure of the inner lamina of the parietes. As already stated, it comes nearer to _B. decorus_ than to any other species. 8. BALANUS AJAX. Pl. 3, fig. 1 _a_-1 _d_. BALANUS TINTINNABULUM (_var._) _Chenu._ Illust. Conch., Tab. 2, fig. 8. _Shell globulo-conical, often elongated in the rostro-carinal axis, pale pink, smooth, extremely massive: parietal pores, close to the basal margin, circular and very small. Scutum with the articular ridge broad and reflexed._ _Hab._--Philippine Archipelago, attached to _Millepora complanata_, Mus. Cuming. Mus. Brit. and Stutchbury.