_Affinities_: in external appearance of the shell, this species can hardly be distinguished from some of the white varieties of _B. amphitrite_; and there is a considerable resemblance, in some of the varieties, in the opercular valves; but the longitudinally striated scuta of _B. eburneus_ suffice to distinguish these certainly very distinct species. Equally, or even more like _externally_, is this species to the _B. Hameri_, so that I have received from an eminent naturalist in the United States both species mingled in the same lot, all bearing the same name of _B. eburneus_; but when the internal structure of the shell is examined, the species are at once seen to be far removed from each other. Still more close is the affinity of this species to _B. improvisus_, both in internal and external characters: it agrees with this species in the singular habit of being able to live in brackish water: these two species are the only ones which have the labrum serrated with teeth, graduated in size, on each side of the central notch. In the case of _young_ specimens of the _var. assimilis_ of _B. improvisus_, an inhabitant of the same seas with _B. eburneus_, the diagnosis is most difficult without long practice; for in the young of _eburneus_, the compartments are only partially covered by yellow epidermis, and have a striped appearance, the radii are sometimes very oblique, the scuta externally have not acquired their longitudinal striæ, and internally the adductor ridge lies not so close to the articular ridge as it does subsequently; hence I for some time mistook the _var. assimilis_ of _B. improvisus_ for the young of _B. eburneus_. But I found in the latter, that the rami of the first pair of cirri, are always, even in the earliest youth, more unequal in length, and that each segment of the posterior cirri bears a greater number of pairs of spines, there being, even in very minute specimens, seven pairs. Moreover, after having examined scores of specimens, I found I could almost always distinguish the two species by the smoothness and curvature of the summits of the radii of _B. improvisus_; I entertain no doubt whatever about the distinctness of the two species; indeed, when both are mature, besides the greater size, striated scuta, &c. of _B. eburneus_, their general aspect is very different. 22. BALANUS IMPROVISUS. Pl. 6, fig. 1 _a_-1 _c_. _Shell white: radii narrow, with their upper margins smooth, slightly arched, very oblique. Tergum with a longitudinal furrow; spur with the end rounded._ _Var._ assimilis, _with longitudinal white hyaline lines_. _Hab._--England, Scotland, Belgium (?), Nova Scotia, United States, West Indies, Rio Plata, Southern Patagonia, Guayaquil, West Colombia; attached to wood, shells, rocks, ships' bottoms, from low tidal level to twenty fathoms depth.