The sutures between the several compartments and the basis are covered by thin membrane, which is continually splitting during the growth of the opposed edges of the underlying shell; but previously to each splitting, a new slip of membrane is, I believe, already formed under the old one; so that the corium is not even momentarily exposed. Owing to this manner of growth, the slips of membrane consist of successive rims united together; in most cases, these soon become abraded from the older parts of the shell, but are sometimes preserved. The last-formed slip of membrane over a suture is homologous with the opercular membrane; and both are strictly analogous with the ring of flexible membrane, forming the joint of the leg of a crab. In the latter case, the flexible membrane and hardened crust are both moulted together: in the opercular membrane, there is a double line of splitting, one close round the opercular valves, and the other at the basal edge of the sheath, and the intermediate portion is moulted, but with a zone of membrane left adherent to the non-moulted valves and sheath: lastly, in the slips of membrane covering the sutures, there is only a single line of splitting, and no portion, I believe, is moulted; the rims of membrane on each side remaining adherent on the compartments and basis, until worn away. The opercular membrane, when closely examined, exhibits no structure, except that it can sometimes be plainly seen to be composed of successive, numerous, excessively thin laminæ. Occasionally, however, it presents the false appearance of being permeated by parallel and anastomosing vessels: this appearance is clue to one or more of the component laminæ having been wrinkled before a succeeding lamina was thrown down and attached to its under side. If a small piece of an opercular valve of Tubicinella, with the opercular membrane adhering to it, and with the corium under both, be dissolved in acid, it may be clearly made out that the corium under the valve has gone on being converted into shell, whereas under the opercular membrane it has been converted and condensed into fine constituent laminæ of chitine. Inasmuch as the successive layers of shell, during each period of growth, go on encroaching on those of the membrane, the line of junction between the shell and chitine becomes oblique or bevelled. The membrane on this bevelled line of junction assumes a slightly different aspect to what it has elsewhere; it becomes yellowish or brown, thicker and very much tougher. In many genera it is also furnished with a row of small bristles. At the period of exuviation the opercular membrane separates just outside this modified portion, leaving the latter adherent, as a rim or slip, on the valves. If, however, the opercular membrane be rudely torn off before its proper period of exuviation, it carries with it the as yet continuous, but already modified, slip. A slightly indented line may sometimes be traced before the period of