The members of this order agree with the Zoantharia sclero-dermata in possessing a well-developed sclerodermic corallum, with a true theca, but generally possessing both tabulae and septa combined. The septa, however, are generally (though apparently not always) some multiple of four, and there is commonly a single predominant septum (sometimes three such), or a vacant space (fossula) representing such a septum. Some of the Rugosa are simple, others are compound; but the latter are destitute of a true coenenchyma. The mode of increase in the compound forms is principally by calicular gemmation, or by lateral budding.
There are only two living genera of corals (viz., the Guynia of the Mediterranean and the Haplophyllia of Florida) which agree with the Rugosa in the tetrameral arrangement of the septa; and it is doubtful whether we are justified in asserting positively on this ground alone that these genera really are Rugose corals. We have, therefore, simply to consider very briefly the corallum of the Rugose corals, which alone has been preserved to us in a fossil condition. In its most essential respects, the corallum of the Rugosa is quite identical with that of the typical Zoantharia sclerodermata. In both alike the corallum may be simple or compound; in both alike the simple form of corallum (fig. 86) consists of an outer wall or "theca," enclosing a central space or "visceral chamber," which is divided into compartments by a series of radiating lamellae or "septa;" in both alike the structures known as " dissepiments," " tabulae," and " columella," may be developed; and in both alike the compound corallum may be regarded as a variously-formed aggregate of "corallites," similar in their fundamental structure to the simple corallum.
On the other hand, the corallum of the Rugosa exhibits the following more striking points of difference as compared with that of the Zoantharia sclerodermata: (1.) The septa appear to be primitively developed in four systems, instead of six or five. Sometimes the adult corallum (as in Stauria) exhibits the four primitive septa in a pre-eminently developed condition, but this is not commonly the case. (2.) The septa are rendered more or less irregular in their arrangement by the presence of a curious vacant space (sometimes three or four), which is known as the "fossula" (fig. 86, B, f), and which appears to take the place of one of the primitive four septa. (3.) When the septa are well developed, they generally present themselves in the adult as of two sizes only, a larger and a smaller (fig. 75, B). (4.) Tabulae are usually present, in conjunction with the septa. (5.) The compound coralla possess no true coenenchyma, and one of their commonest modes of increase is by means of "calicular gemmation."
Fig. 86. - Morphology of the Rugosa. A, Fragment of Zaphrentis gigantea, showing the septa (s) with the sparse dissepiments crossing the interseptal loculi, the epitheca (e), and the thin proper wall (w). B, Transverse section of Zaphrentis Guerangeri, showing the septa and dissepiments, the central area occupied solely by the tabulae, and the "fossula" (f). C, Longitudinal section of the last, showing the arrangement of the tabulae. (A is after Edwards and Haime; B and C are after James Thomson.)
Recently it has been shown that some very abnormal Rugose corals were provided with a lid or operculum, closing the mouth of the calice. In the genus Calceola (fig. 88), formerly referred to the Brachiopoda, and very abundant in certain parts of the Devonian system, the operculum consisted of a single valve or piece. In Goniophyllum four valves were present, and in Cystiphyllum prismaticum there were four or more valves in the operculum. It is worthy of notice that some recent corals (species of Primnoa, Paramuricea, and others) exhibit also a more or less complete operculum. The calices of Cryptohelia pudica (one of the Hydroid group of the Stylaster-idae) are also protected by a calcareous lamina in front of each.
According to Professor Agassiz, the Rugosa ought not to be considered as belonging to the Actinozoa, but should be placed amongst the Hydrozoa. This radical change cannot, however, be accepted unless upon the production of much more evidence than has yet been brought forward in its favour. One strong argument against this view is to be found in the fact that the typical Rugose corals possess well-developed septa - structures which, if they do not absolutely imply the existence of mesenteries, are, at any rate, unknown in any living Hydro-zoon. At present it is not possible to speak definitely as to the systematic position of the Rugosa, but they appear to form a natural and distinct group, intermediate in many respects between the Zoantharia and the Alcyonaria.
Fig. 87. - Strombodes pentagonus. A Silurian Rugose Coral.
Fig. 88. - Calceola sandalina. An operculate Rugose Coral. Devonian.
The Rugosa are divided into the following families:
1. Stauridae: Septa well developed, extending from the bottom to the top of the visceral chamber, and showing a conspicuous quaternary arrangement. Dissepiments are present, and there is a central tabulate area. Genera - Stauria, Polycaelia, Metriophyllum, Holo-cystis, Conosmilia.
2. Cyathaxonidae: Corallum simple, with a deep calice; septa well developed, the four primary septa not predominantly developed; no dissepiments or tabulae. Genera - Cyathaxonia, Guynia, Haplo-phyllia.
3. Cyathophyllidae: Corallum simple or compound; septa well developed, but not so completely so as in the two preceding groups; the four primitive septa not pre-eminently developed ; tabulae always, and dissepiments generally, present. Genera - Zaphrentis, Amplex-us, Cyathophyllum, Heliophylhwi, Omphyma, Lithostrotion, Lons-daleia, Clisiophyllum, etc.
4. Cystiphyllidae : Corallum simple or rarely compound ; wall complete; septa rudimentary; visceral chamber with small convex vesicles formed by a combination of tabulae and dissepiments; sometimes an operculum. Genera - Cystiphyllum, Goniophyllum, Rhizophyllum, Calceola.