No true Graptolites have ever been detected in strata of Devonian age; and the whole of this group has become extinguished—­unless we refer here the still surviving Dictyonemoe.  The Coelenterates, however, are represented by a vast number of Corals, of beautiful forms and very varied types.  The marbles of Devonshire, the Devonian limestones of the Eifel and of France, and the calcareous strata of the Corniferous and Hamilton groups of America, are often replete with the skeletons of these organisms—­so much so as to sometimes entitle the rock to be considered as representing an ancient coral-reef.  In some instances the Corals have preserved their primitive calcareous composition; and if they are embedded in soft shales, they may weather out of the rock in almost all their original perfection.  In other cases, as in the marbles of Devonshire, the matrix is so compact and crystalline that the included corals can only be satisfactorily studied by means of polished sections.  In other cases, again, the corals have been more or less completely converted into flint, as in the Corniferous limestone of North America.  When this is the case, they often come, by the action of the weather, to stand out from the enclosing rock in the boldest relief, exhibiting to the observer the most minute details of their organization.  As before, the principal representatives of the Corals are still referable to the groups of the Rugosa and Tabulata.  Amongst the Rugose group we find a vast number of simple “cup-corals,” generally known by the quarrymen as “horns,” from their shape.  Of the many forms of these, the species of Cyathophyllum, Heliophyllum (fig. 82), Zaphrentis (fig. 81), and Cystiphyllum (fig. 80), are perhaps those most abundantly represented—­none of these genera, however, except Heliophyllum, being peculiar to the Devonian period.  There are also numerous compound Rugose corals, such as species of Eridophyllum, Diphyphyllum, Syringopora, Phillipsastroea, and some of the forms of Cyathophyllum and Crepidophyllum (fig. 83).  Some of these compound corals attain a very large size, and form of themselves regular beds, which have an analogy, at any rate, with existing coral-reefs, though there are grounds for believing that these ancient types differed from the modern reef-builders in being inhabitants of deep water.  The “Tabulate Corals” are hardly less abundant in the Devonian rocks than the Rugosa; and being invariably compound, they hardly yield to the latter in the dimensions of the aggregations which they sometimes form.

[Illustration:  Fig. 84.—­Portion of a mass of Favosites Gothlandica, of the natural size.  Upper Silurian and Devonian of Europe and America. (Original.) Billings.]

[Illustration:  Fig. 85.—­Fragment of Favosites hemispherica, of the natural size.  Upper Silurian and Devonian of America.  (After Billings.)]