FAM. 2. PRIMNOIDAE.—This is a well-marked family. The axis of the colonies is horny and calcareous. The coenenchym and the non-retractile zooids are protected by scale-like spicules, which usually overlap and form a complete armour for the protection of the soft parts. On the aboral side of the base of each tentacle there is a specialised scale, and these fit together, when the tentacles are folded over the peristome, to form an operculum. The principal genera are:—_Primnoa_, Lamouroux; Atlantic Ocean, occurring also in the Norwegian fjords. This genus is usually found in moderately deep water, 100 to 500 fathoms. _Primnoella_, Gray. This genus seems to be confined to the temperate seas of the southern hemisphere. It is unbranched. The zooids are arranged in whorls round the long whip-like stem. _Plumarella_, Gray; southern hemisphere, in moderately deep water. This is branched pinnately in one plane. The zooids are small and arise at considerable intervals alternately on the sides of the branches. _Stenella_, Gray; widely distributed in deep water. The zooids are large and are arranged in whorls of three situated at considerable distances apart. _Stachyodes_, W. and S.; Fiji, Kermadecs, Azores, in deep water. Colony feebly branched. Zooids in regular whorls of five. Other genera belonging to this group of Primnoidae are _Thouarella_, Gray, and _Amphilaphis_, Antarctic seas. The following genera are placed in separate sub-families:—_Callozostron_, Wright; Antarctic Sea, 1670 fathoms. The axis is procumbent and the zooids are thickly set in rows on its upper surface. The zooids are protected by large imbricate scales, of which those of the last row are continued into long spine-like processes. _Calyptrophora_, Gray; Pacific Ocean, in deep water. The base of the zooids is protected by two remarkably large scales. _Primnoides_, W. and S.; Southern Ocean. The opercular scales are not distinctly differentiated and the calyx is therefore imperfectly protected. {355}FAM. 3. CHRYSOGORGIIDAE.[384]—The axis in this family is composed of a horny fibrous substance with interstratified calcareous particles, and it springs from a calcareous plate, which sometimes gives off root-like processes. It may be unbranched or branched in such a way that the branches of the second, third, and subsequent orders assume in turn the direction of the base of the main axis. The axis is frequently of a metallic iridescent appearance. The zooids usually arise in a single straight or spiral row on the branches, and are not retractile. The coenenchym is thin. The spicules vary considerably, but in a very large proportion of the species they are thin, oval, or hour-glass plates (Fig. 149, 10, p. 336).