The _Scuta_ differ from those of _B. psittacus_ only in the basi-tergal corner not being so much rounded off, and consequently in the articular ridge, which is rather more reflexed, descending in proportion lower down the valve: the cavity at the basi-tergal corner is in proportion broader. The valves in the two species differ, also, but only in young specimens, in the occludent half being tinted, both externally and internally, purple, whereas in _B. psittacus_ the whole valve, at all ages, is white. In the _terga_ the spur is removed fully its own width from the basi-scutal angle, whereas it is not half this distance in _B. psittacus_. The scutal margin is here much more inflected. In _B. psittacus_ there is a remarkable patch of purple on the inside of the valve, between the articular ridge and a second special ridge; of this purple patch there is here no trace, consequently the beak or apex is white. The beak, also, is less prominent. The special ridge, just alluded to, here runs much nearer to the articular ridge, and is less prominent: indeed, in old specimens, it is often almost obliterated. Finally, the whole valve, in proportion to the Scutum, is rather broader. I have seen a young specimen, about a quarter of an inch in basal diameter, with the orifice of the shell toothed owing to the obliquity of the summits of the radii; and this gave the shell a very peculiar aspect. The largest well-coloured specimen which I have seen is 1.2 of an inch in basal diameter; but in Mr. Cuming's collection there are two rugged, disintegrated specimens, two inches in basal diameter, and two and a half in height. Some specimens, 1.3 in basal diameter, in Mr. Stutchbury's collection, are remarkable from the radii having been obliterated--the shell being merely divided by six sutures, as we have seen is likewise sometimes the case with large specimens of _B. psittacus_. This species is evidently a South African representative of the South American _B. psittacus_. 5. BALANUS NIGRESCENS. Pl. 2, fig. 5 _a_, 5 _b_. BALANUS NIGRESCENS. _Lamarck_, (1818) in _Chenu_. Illust. Conch., Tab. 4, fig. 16. ---- GIGAS. _Ranzani._ Memoire di Storia Nat., 1820, Tab. 3, fig. 5, 6, 7. ---- ---- _De Blainville._ Dict. des Sc. Nat., Tab. 116, fig. 2, 2 _a_. _Shell cinereous, tinted with pale or blackish blue, or wholly white. Scutum with the articular ridge terminating downwards in a small, sharp, free point: adductor ridge prominent. Tergum with the apex produced and needle-like._ _Hab._--Swan River, West Australia, Mus. Brit., attached to sandstone. Attached to sandstone and to each other in a group, Mus. Cuming. Twofold Bay, S. E. Australia, attached to tidal rocks and Patellæ, Mus. Darwin.