FAM. 2. HELIOPORIDAE.[372]—Coenothecalia with small irregularly arranged coenosarcal caeca, and a variable number of septa or septal ridges. _Heliopora_, de Blainville, recent, Eocene and Upper Cretaceous. _Polytremacis_, d'Orbigny, Eocene and Upper Cretaceous. _Octotremacis_, Gregory, Miocene. The family COCCOSERIDAE is regarded by Lindström as a sub-family of the Heliolitidae, and the families THECIDAE and CHAETETIDAE are probably closely related to the Helioporidae. ORDER III. ALCYONACEA. This order contains a large number of genera of great variety of form. The only characters which unite the different genera are that the body-walls of some groups of zooids, or of all the zooids, are fused together to form a common coenenchym penetrated by the coenosarcal canals, and that the spicules do not fuse to form a solid calcareous, or horny and calcareous, axial skeletal support. The affinities with the order Stolonifera are clearly seen in the genera _Xenia_ and _Telesto_. Some species of _Xenia_ form flattened or domed colonies attached to stones or corals, with non-retractile anthocodiae and body-walls united for only a {347}short distance at the base. Young _Xenia_ colonies are in fact Stolonifera in all essential characters. In _Telesto prolifera_ we find a network of stolons encrusting coral branches and other objects after the manner of the stolons of many species of _Clavularia_, although the zooids do not arise from these stolons singly, but in groups, with their body-walls fused together for a certain distance. In _Telesto rubra_ the spicules of the body-walls are fused together to form a series of perforated tubes very similar in some respects to the tubes of _Tubipora_. A remarkable genus is _Coelogorgia_. Here we find a branching colony arising from a basal stolon, and the axis of the main stem and of each branch consists of a single very much elongated zooid bearing on its thickened walls the branches of the next series and other zooids. It is true that in this genus there is very little fusion of neighbouring zooids, and the amount of true coenenchym is so small that it can hardly be said to exist at all. Bourne[373] has united this genus with _Telesto_ into a family Asiphonacea, which he joins with the Pennatulida in the order Stelechotokea; but their affinities seem to be closer with the Alcyonacea than with the Pennatulacea, from which they differ in many important characters. [Illustration: FIG. 154.—_Alcyonium digitatum_, a single-lobed specimen, with some of the zooids expanded.] The genus _Alcyonium_ not only contains the commonest British Alcyonarian (_A. digitatum_), but it is one of the most widely distributed genera of all Alcyonaria that occur in shallow water.