overlying both; and (_z_, _z_) the layers of cement, which may for the present be disregarded. In order to allow, of the growth of the shell, the circumferential slip (_b_) periodically splits in the middle, all round, in a line exactly conformable to the edge of the last-formed slip of basal membrane; and likewise in straight, medial lines under the spoke-like (cut off in fig. 1 _a_) doubled walls. I have seen, under a high power, the line of splitting, very shortly after its formation, with the two edges ragged and near together, with an extremely narrow, new circumferential slip just formed, between and over the edges of the previously formed slip. What causes the circumferential slip to split so symmetrically, I cannot say: the opposed edges, after a time, become thickened, apparently from adhering to the underlying layer of cement, as will presently be described. The circumferential slip continues increasing in breadth till the period of its splitting arrives, by which time it has become much broader than the last-formed slip of basal membrane; and after the splitting takes place, the interior half towards the basal membrane, forms a new basal slip all round the basis, and the exterior half adds a new slip to the membrane investing the walls. This latter membrane being inflected under the basal edges of the walls, is, during the growth of their edges, drawn straight down, the newly-formed portion taking the inflected position. In the sectional diagram, (1 _b_) the circumferential slip is not yet broad enough to split; when it has become so, it will split under the letter (_b_). The slips of basal membrane are, as may be seen in fig. 1 _a_, narrower towards the circumference; but the two or three last-formed slips, are out of proportion narrower than the others; and it is certain, from the comparison of the basal membranes of specimens of different ages, that these will afterwards increase in width.[71] I have seen no other instance, in Cirripedes, of growth in membranes, except at their extreme margins: I suspect that these last-formed slips are pulled, during the downward and outward growth of the shell, a little from over the last-formed slips, new and larger laminæ being all the time thrown down, so as to prevent any fissure being formed. I also suspect that the gradual increase in width of the circumferential slip itself, is due to the opposed edges of the underlying and last-formed circumferential slip being dragged further apart from each other, new and wider laminæ of membrane being continually thrown down; the new circumferential slip being thus, also, all the time thickened, as well as rendered broader.