The sub-kingdom of the _Coelenterates_, so far as certainly known, is represented only by _Corals_;[19] but the remains of these are so abundant in many of the limestones of the Carboniferous formation as to constitute a feature little or not at all less conspicuous than that afforded by the Crinoids. As is the case in the preceding period, the Corals belong, almost exclusively, to the groups of the _Rugosa_ and _Tabulata_; and there is a general and striking resemblance and relationship between the coral-fauna of the Devonian as a whole, and that of the Carboniferous. Nevertheless, there is an equally decided and striking amount of difference between these successive faunas, due to the fact that the great majority of the Carboniferous _species_ are new; whilst some of the most characteristic Devonian _genera_ have nearly or quite disappeared, and several new genera now make their appearance for the first time. Thus, the characteristic Devonian types _Heliophyllum, Pachyphyllum, Chonophyllum, Acervularia, Spongophyllum, Smithia, Endophyllum_, and _Cystiphyllum_, have now disappeared; and the great masses of _Favosites_ which are such a striking feature in the Devonian limestones, are represented but by one or two degenerate and puny successors. On the other hand, we meet in the Carboniferous rocks not only with entirely new genera--such as _Axophyllum, Lophophyllum_, and _Londsdaleia_--but we have an enormous expansion of certain types which had just begun to exist in the preceding period. This is especially well seen in the Case of the genus _Lithostrotion_ (fig. 116, b), which more than any other may be considered as the predominant Carboniferous group of Corals. All the species of _Lithostrotion_ are compound, consisting either of bundles of loosely-approximated cylindrical stems, or of similar "coral-lites" closely aggregated together into astræiform colonies, and rendered polygonal by mutual pressure. This genus has a historical interest, as having been noticed as early as in the year 1699 by Edward Lhwyd; and it is geologically important from its wide distribution in the Carboniferous rocks of both the Old and New Worlds. Many species are known, and whole beds of limestone are often found to be composed of little else than the skeletons of these ancient corals, still standing upright as they grew. Hardly less characteristic of the Carboniferous than the above is the great group of simple "cup-corals," of which _Clisiophyllum_ is the central type. Amongst types which commenced in the Silurian and Devonian, but which are still well represented here, may be mentioned _Syringopora_ (fig. 116, e), with its colonies of delicate cylindrical tubes united at intervals by cross-bars; _Zaphrentis_ (fig. 116, d), with its cup-shaped skeleton and the well-marked depression (or "fossula") on one side of the calice; _Amplexus_ (fig. 116, c), with its cylindrical, often irregularly swollen coral and short septa; _Cyathophyllum_ (fig.