By some authors this family is considered to be the simplest and most primitive of the Axifera; but the delicate character of the axis of the main stem and branches, the thinness of the coenenchym, the position of the zooids on one side of the branches only, and the tenuity of the calcareous spicules may be all accounted not as primitive characters, but as special adaptations to the life in the slow uniform currents of deep water. The principal genera are:—_Lepidogorgia_, Verrill; Atlantic and Pacific Oceans, 300 to 1600 fathoms. Axis unbranched. Zooids large and arranged in a single row. _Trichogorgia_, Hickson; Cape of Good Hope, 56 fathoms. Colony branching in one plane. Zooids numerous and on all sides of the branches. _Chrysogorgia_, D. and M.; deep water. Axis branched. Spicules on the zooids always large. _Metallogorgia_, Versluys; Atlantic Ocean, 400 to 900 fathoms. Basal part of the stem unbranched (monopodial). _Iridogorgia_, Verrill. Spiral stem and branches. _Pleurogorgia_, Versluys. Axis branched in one plane. Coenenchym thick. _Riisea_, D. and M. Monopodial stem and thick coenenchym. FAM. 4. MURICEIDAE.—This is a large family, exhibiting very great variety of habit. The spicules are often very spiny, and project beyond the surface of the ectoderm, giving the colony a rough appearance. A great number of genera have been described, but none of them are very well known. The family requires careful revision. The more important genera are:—_Acanthogorgia_, Gray; principally in deep water in the Atlantic Ocean. The calices are {356}large, cylindrical, and spiny. _Villogorgia_, D. and M.; widely distributed. Delicate, graceful forms, with thin coenenchym. _Echinomuricea_, Verrill; _Muricea_, Lamouroux; _Paramuricea_, Köll; _Acamptogorgia_, W. and S.; _Bebryce_, Philippi. FAM. 5. PLEXAURIDAE.—In this family we find some of the largest and most substantial Gorgonids. The axis is usually black, but its horny substance may be impregnated with lime, particularly at the base. The coenenchym is thick, and the zooids are usually completely retractile, and the surface smooth. The species of the family are principally found in shallow water in warm or tropical regions. The principal genera are:—_Eunicea_, Lamouroux. The calices are prominent, and not retractile. _Plexaura_, Lamouroux; _Euplexaura_, Verrill. _Eunicella_, Verrill. With an outer layer of peculiar torch-shaped spicules. The only British species of this order is _Eunicella cavolini_ (formerly called _Gorgonia verrucosa_). It is found in depths of 10 to 20 fathoms off the coast of the English Channel and west of Scotland. Occasionally specimens are found in which a gall-like malformation with a circular aperture is seen, containing a Barnacle. Such gall formations, common enough in some species of Madreporaria, are rarely found in Alcyonaria. [Illustration: FIG. 156.—_Eunicella cavolini._ Some branches of a large dried specimen, showing a gall formed by a Cirripede.]