In the present state of our knowledge the Zoantharia in which a primary cycle of six couples of mesenteries is (or may be inferred to be) completed by the addition of two pairs to the eight Edwardsian mesenteries, and succeeding cycles are formed in the exocoeles of the pre-existing mesenterial cycles, may be classed in an order Actiniidea, and this may be divided into the suborders Malacactiniae, comprising the soft-bodied Actinians, such as Actinia, Sagartia, Bunodes, &c., and the Scleractiniae, comprising the corals. The Scleractiniae may best be divided into groups of families which appear to be most closely related to one another, but it should not be forgotten that there is great reason to believe that many if not most of the extinct corals must have differed from modern Actiniidea in mesenterial characters, and may have only possessed Edwardsian mesenteries, or even have possessed only four mesenteries, in this respect showing close affinities to the Stauromedusae. Moreover, there are some modern corals in which the secondary cycle of mesenteries departs from the Actinian plan. For example, J.E. Duerden has shown that in Porites the ordinary zooids possess only six couples of mesenteries arranged on the Actinian plan. But some zooids grow to a larger size and develop a number of additional mesenteries, which arise either in the sulcar or the sulcular entocoele, much in the same manner as in Cerianthus. Bearing this in mind, the following arrangement may be taken to represent the most recent knowledge of coral structure:—
Group A.
Family I. Zaphrentidae.—Solitary Palaeozoic corals with an epithecal wall. Septa numerous, arranged pinnately with regard to four principal septa. Tabulae present. One or more pits or fossulae present in the calicle. Typical genera—Zaphrentis, Raf. Amplexus, M. Edw. and H. Streptelasma, Hall. Omphyma, Raf.
Family 2. Turbinolidae.—Solitary, rarely colonial corals, with radially arranged septa and without tabulae. Typical genera— Flabellum, Lesson. Turbinolia, M. Edw. and H. Caryophyllia, Lamarck. Sphenotrochus, Moseley, &c.
Family 3. Amphiastraeidae.—Mainly colonial, rarely solitary corals, with radial septa, but bilateral arrangement indicated by persistence of a main septum. Typical genera—Amphiastraea, Étallon. Thecidiosmilia.
Family 4. Stylinidae.—Colonial corals allied to the Amphiastraeidae, but with radially symmetrical septa arranged in cycles. Typical genera—Stylina, Lamarck (Jurassic). Convexastraea, D’Orb. (Jurassic). Isastraea, M. Edw. and H.(Jurassic). Ogilvie refers the modern genus Galaxea to this family.
Group B.
Family 5. Oculinidae.—Branching or massive aporose corals, the calices projecting above the level of a compact coenenchyme formed from the coenosarc which covers the exterior of the corallum. Typical genera—Lophohelia, M. Edw. and H. Oculina, M. Edw. and H.
Family 6. Pocilloporidae.—Colonial branching aporose corals, with small calices sunk in the coenenchyme. Tabulae present, and two larger septa, an axial and abaxial, are always present, with traces of ten smaller septa. Typical genera—Pocillopora, Lamarck. Seriatopora, Lamarck.
Family 7. Madreporidae.—Colonial branching or palmate perforate corals, with abundant trabecular coenenchyme. Theca porous; septa compact and reduced in number. Typical genera— Madrepora, Linn. Turbinaria, Oken. Montipora, Quoy and G.