the part formerly occupied by the pulp. In the section of a newly formed part, the space between the surfaces of the ring is of a very delicate and spongy character, of equal consistence throughout, and very flexible. In a similar section of older parts, the texture is firmer and more solid, and the inner half of the space more compact, and of a deeper colour than the external half. In the lowest portions the structure is very solid and unyielding, so that, in old specimens, it cannot be bent without the danger of breaking. In the Sertularia argentea the youngest parts are of a silvery whiteness, which in the older, changes to a brown colour. This change is not produced by incrustations on the external surface, or by the transudation of foreign matter from without, but by an organic change in the sheath itself; since it begins from the interior surface and proceeds to the exterior, and has regular gradations from the newest part, where it is nearly absent, to the older portions. In the pheasant's tail coralline (Plumularia myriophyllum), the polypidom is profusely marked with oval orifices, very similar in appearance to the stomata of flowering plants. This shows a structure very different, and far superior to what would be expected from a mere exudation from the pulp. In the spaces between the enlargement of the stem and the ingrafting of the internode, the surface is striated, as if traversed by vessels: but I have never detected any vascular appearance in this or any other species. It is very common to observe, in many species of Sertulariadae, that the lower pinnae are frequently absent, but leaving marks behind in proof of their once existing. This is most remarkably the case in Thuiaria thuja and Thu. articulata, but is observable in many others, as the Plumularia falcata, Plu. setacea, &c.: it is, no doubt, in a great measure produced by the violence of the waves, but not entirely so. In examining some specimens, when taken immediately from the water, the central pulp of the lower pinnae and branches is frequently pale, and the polypes appear inactive and pale, as if diseased. If this is very much the case, a well-defined line of demarcation will be observed at that part of the branch which is inserted into the trunk this line appears also on the horny sheath, and the branch is sloughed off, apparently in the same manner that trees shed their leaves. After the part has fallen, the truncated surface becomes contracted, the scar healed, and the exposed pulp shielded from contact with the water: this is no doubt greatly assisted by the action of the waves, though it does not seem the great cause of it: if it was caused entirely by the motion of the sea, those species which grow near the shores would be the most liable to it, from being more exposed. In
