THE Medusa mentioned by Mr. Archer in NATURE, vol. xxiv. p. 307, is undoubtedly Polydonia frondosa, Ag., figured in the Contributions to the Natural History of the United States. This Medusa was already known to Pallas, who de scribed alcoholic specimens sent him from the West Indies by Drury. It is stated by Agassiz to be quite common along the Florida Keys. I have myself observed it in great abundance at the Tortugas, in the moat of Fort Jefferson, and in the mud flats to the north of Key West. They occur there in from three to six feet of water, the disk resting upon the bottom, the tentacles turned upwards; the disk pulsates slowly while they are at rest. Their habits when disturbed are well described by Mr. Archer. The young sometimes swim near the surface, and are far more active than larger specimens. When kept in confinement they also creep slowly over the ground by means of their tentacles, or, raising themselves sometimes edgeways against the sides of the dishes, remain stationary for a considerable time. The resemblance of Polydonia when at rest upon the bottom to large Actiniæ with fringed tentacular lobes, such as Phythactis, is very striking. The peculiar habits of Polydonia were noticed by Mertens in a species named by Brandt P. Mertensii in 1838; and found at the Carolines. The genus Polydonia was established by Brandt, and not by Agassiz, as is stated by Haeckel in his “System der Medusen”.