I will now proceed to describe, in some detail, the cementing apparatus in the several following genera. [70] I had some slight reason to suspect in Pollicipes that new cement-glands were successively formed: this is more probable in this genus than in the others, inasmuch as it is the most nearly related to the Balanidæ. _Coronula._--The cementing apparatus is here more simple than in any other genus of the Balaninæ, and I have studied it more carefully. The basal membrane of _Coronula balænaris_ is figured in Pl. 28, fig. 1 _a_, and must first be described; its relation to the shell will hardly be understood without looking at the outline of the folded walls of this species, in Pl. 16, fig. 5. The basal membrane closes the central circular hollow, and is continuous with rays (not represented in Pl. 28) extending under the doubled walls and terminal transverse loops. It has eighteen concave sides, corresponding with the inner ends of the folded walls, for each of the six compartments is trebly folded. The membrane consists of successive, conformable slips (_c′_, _c′_), bordered exteriorly by thickened yellowish rims, and internally overlapping (when viewed from the inner side) the few last-formed slips, and then thinning out. The membrane forming each slip is itself laminated. The middle portion, about 1/50th of an inch in diameter, is rather opaque, owing to the slips being so close together. Beyond this central part, the slips suddenly increase in size, but yet have a different shape from the 18-sided outline, which they ultimately assume: this difference is owing to the great changes in shape, as explained under the genus Coronula, which the shell undergoes, when the walls at first assume their folded structure. The walls are invested by longitudinally striated membrane (_p_, _p_, _p_, fig. 1 _a_), which turns in under their basal edges; and this membrane is united with the basal membrane, by what I shall call the circumferential slip (_b_), and which is shaded in fig. 1 _a_, simply for the sake of catching the eye. It is the circumferential slip of membrane which sends rays under the spoke-like folded walls: thin as it is, this slip is yet laminated, but is not bordered by thickened edges. The membrane investing the walls is, like the basal membrane, formed of successive slips with thickened edges, which overlapping (viewed from the inside) the last-formed slips, project beyond them, and so face the edges of the slips in the basal membrane; they are only obscurely indicated in fig. 1 _a_. The circumferential slip (_b_) lies over (as viewed from within) both the basal and wall membrane. This whole structure will, perhaps, be best understood by the sectional diagram (fig. 1 _b_), in which the letters (_c′_, _c′_) show the slips of basal membrane; (_p_) the parietal membrane, coating the outside surface of the walls of the shell, not here represented; (_b_) the circumferential slip