outer edge of the “barrier-reefs” which encircle the Society Islands. [13] Kotzebue’s “First Voyage,” vol. iii, p. 142. Near Porto Praya, in the Cape de Verde Islands, some basaltic rocks, lashed by no inconsiderable surf, were completely enveloped with a layer of Nulliporæ. The entire surface over many square inches, was coloured of a peach-blossomed red; the layer, however, was of no greater thickness than paper. Another kind, in the form of projecting knobs, grew in the same situation. These Nulliporæ are closely related to those described on the coral-reefs, but I believe are of different species. [14] Kotzebue’s “First Voyage,” vol. ii, p. 16. Lieutenant Nelson, in his excellent memoir in the Geological Transactions (vol. ii, p. 105), alludes to the rocky points mentioned by Kotzebue, and infers that they consist of Serpulæ, which compose incrusting masses on the reefs of Bermudas, as they likewise do on a sandstone bar off the coast of Brazil (which I have described in _London Phil. Journal,_ October 1841). These masses of Serpulæ hold the same position, relatively to the action of the sea, with the Nulliporæ on the coral-reefs in the Indian and Pacific Oceans. [15] Captain Moresby, in his valuable paper “on the Northern atolls of Maldivas” (_Geographical Journal_, vol. v), says that the edges of the reefs there stand above water at low spring-tides. There appears to be scarcely a feature in the structure of Keeling reef, which is not of common, if not of universal occurrence, in other atolls. Thus Chamisso describes[16] a layer of coarse conglomerate, outside the islets round the Marshall atolls which “appears on its upper surface uneven and eaten away.” From drawings, with appended remarks, of Diego Garcia in the Chagos group and of several of the Maldiva atolls, shown me by Captain Moresby,[17] it is evident that their outer coasts are subject to the same round of decay and renovation as those of Keeling atoll. From the description of the atolls in the Low Archipelago, given in Captain Beechey’s “Voyage,” it is not apparent that any conglomerate coral-rock was there observed. [16] Kotzebue’s “First Voyage,” vol. iii, p. 144. [17] See also Moresby on the Northern atolls of the Maldivas, _Geographical Journal_, vol v, p. 400.