I find, in most collections, this species confounded with _B. balanoides_; I have even seen the two species, placed by Leach, on the same tablet in the British Museum: _B. balanoides_ is, moreover, generally confounded with _Chthamalus stellatus_; nor has any one hitherto separated the present species from _B. improvisus_. On the other hand, trifling varieties, both of _B. balanoides_ and _B. crenatus_, have commonly been considered as specifically distinct. From these facts it will be seen in what confusion our commonest British species of Balanus have been left. After due deliberation, I have little doubt that this is the _B. crenatus_ of Bruguière, and probably the _B. rugosus_ of Montagu, but this latter author omits all reference to the really important diagnostic characters between this species and _B. balanoides_. The _B. crenatus_ is certainly the _B. rugosus_ of Dr. Aug. Gould. In various collections, I find specimens of _B. crenatus_, when coming from the arctic regions, called _B. glacialis_, _arcticus_, and _borealis_; though I have not met with an authentic specimen of the _B. glacialis_ of Gray ('Supp. Parry's Voyage,' 1819, p. ccxlvi), I have little doubt that it would prove to be the present species. _General Appearance._--White, usually of a dirty tint, from the yellowish or brownish persistent epidermis: conical, generally (fig. 6 _a_) with the parietes rugged and irregularly folded longitudinally; but sometimes much depressed and extremely smooth (6 _b_); often cylindrical and very rugged; occasionally club-shaped (6 _c_), the upper part being much wider than the lower: specimens in this latter condition sometimes have extremely narrow parietes, like mere ribs, and wide radii. The orifice in the cylindrical varieties is often most deeply toothed. The radii are generally narrow, and have jagged oblique summits; but not infrequently they are so narrow as to form mere linear borders to the compartments. The orifice is rhomboidal, passing into oval, either very deeply or very slightly toothed.