[Illustration:  Fig. 116—­Corals of the Carboniferous Limestone. a. Cyathophyllum paracida, showing young corallites budded forth from the disc of the old one; a’, One of the corallites of the same, seen in cross-section; b, Fragment of a mass of Lithostrotion irregulare; b’, One of the corallites of the same, divided transversely; c, Portion of the simple cylindrical coral of Amplexus coralloides; c’, Transverse section of the same species; d, Zaphrentis vermicularis, showing the depression or “fossula” on one side of the cup; e, Fragrent of a mass of Syringopora ramulosa; f, Fragment of Coetetes tumidus; f’, Portion of the same of the same, enlarged.  From the Carboniferous Limestone of Britain and Belgium. (After Thomson, De Koninck, Milne-Edwards and Haime, and the Author.)]

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 astraeiform 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