I have as yet only alluded to the _circumferential duct_ (_i_, _i_, Pl. 28, fig. 4 _a_): we have not hitherto met with this duct, but I suspect that the branches which in Chelonobia inosculate, and which seem to run nearly parallel to the circumference of the basal membrane, answer the same purpose of connecting the ducts together, and are, perhaps, strictly homologous. In this, and some other species of Balanus, the last-formed circumferential duct runs round the margin of the upper lamina of the basal plate, close to the basal edges of the walls; and as these latter have projecting longitudinal ribs, the duct curves a little round each rib; so that the whole duct is formed by as many short inwardly curved portions as the walls have ribs, or longitudinal septa. Between the basal extremities of these parietal, longitudinal septa, the extremities of the radiating septa of the basis project and enter; and along the crests of the latter, little branch-ducts (_i′_), proceeding from the circumferential duct, extend. In the basis, beneath the tubes formed by the just-mentioned radiating septa, there is a cancellated shelly mass (which, in fig. 4 _a_, was of unusual thickness), and along the crests of the branching ridges forming this cancellated mass, the sub-branches of the above branch-ducts (_i′_) run; these soon become so minute as not to be distinguished by the highest powers, and thus form a sheet of cement, which attaches the last-formed zone of the shelly basis to the supporting surface. At what point the membrane forming any one duct ceases, the cement-tissue being alone left, I was not able to ascertain; but the lower parts of the reticulated slip (_z_, _z_, fig. 4 _a_) closely resembled the cement-tissue which surrounds the disc-segment of the pupal antennæ in _Lepas australis_. The circumferential duct, here and there, forms little loops, as may be seen in fig. 4 _a_: and often two branches, running along the crests of two adjoining basal septa, proceed from a common point of the circumferential duct. The cement itself, under different parts of the basis, appears as little separate discs, as threads, globules, and as a fine network, but most commonly as simple layers. As each thick zone of shelly matter is added round the basis, the exterior branches of the ducts, between the circumferential duct and the new layer of cement beneath, are fairly imbedded in shell, and are for ever hidden, without, indeed, acid be used for the dissolution of the calcareous matter: so, also, the pre-existing ducts and glands, and the main trunk, would all have been hidden, if the layer of calcareous matter, which, I believe, is thrown down at each period of growth over the entire surface, had not been of excessive tenuity.