328 FOSSIL INVERTEBRATES/Corals and Other Cnidaria contain both solitary and modular growth forms All tabulate corals are modular colonies, as are all of the accepted Cambrian corals and the Numidiaphyllida Overall, solitary corals show a wide variety in form, but this may be consistently developed within many species Discoidal (Figures 8B and 10A–C) and similar broad, flat growth forms (tympanoid, cupolate) and those with a noncircular cross-section (calceoloid, flabellate, cuneiform, etc.) tend to be the least variable at the species level Conicocylindrical solitary corals (Figure 8C) may show more irregular forms as the result of disturbance during growth, at least among the Rugosa where adult solitary corals are generally free-living Colonial growth form in the skeleton varies from branching to massive, ramose, and foliose In many branching (fasciculate) forms, the polyps separate completely, and the clonal group is held together only by the corallites they secrete beneath themselves (Figure 8H and 8I) The cluster of corallites may be loose and divergent (dendroid; Figure 10E) or close and parallel (phaceloid), and the arrangement appears to be under genetic constraint in many cases Corallites may remain attached on opposite sides, like the palisades in a fence, the ranks joined together to enclose lacunae in the skeleton (cateniform; Figure 9G and 9H) An external wall may enclose the ranks but along them adjacent corallites may be in contact or separated by colonial tissue, suggesting neural connectivity between the polyps A range of structural relationships between the modules exist in massive, ramose, and foliose growth forms Corallites may be packed more or less tightly together, becoming polygonal in cross-section, each retaining their own wall (rugosan cerioid, suggesting complete separation of the polyps; Figure 8J–M) However, various levels of integration of the polyps may be reflected in the loss of individual corallite walls (for example scleractinian cerioid, Figure 10D) Figure Representative Rugose corals (A) Amplexizaphrentis sp showing a pinnate arrangement of septa reflecting the pattern of insertion; Lower Carboniferous, Dinantian, Northern England, UK; Â1.5 (B) Palaeocyclus porpita, discoidal corallum showing a pinnate arrangement of septa particularly about the very short cardinal septum, bottom centre; lower Silurian, Llandovery Series, Gotland, Sweden Â2.5 (C) Typical solitary horn coral, shown in growth orientation; Carboniferous, locality unknown; Â2 (D, E) Tabulophyllum sp., showing well developed septa, dissepiments, and tabulae; Upper Devonian, Frasnian, East Ogwell Limestone, south Devon, UK; Â1.5: (D) cross section and (E) longitudinal section (F, G) Arachnophyllum murchisoni, an amural colony with (G) pseudo walls formed of stacks of dissepiments; Silurian, Much Wenlock Limestone, Shropshire, UK; Â1: (F) cross section and (G) longitudinal section (H, I) Lonsdaleia duplicata, phaceloid fasciculate colony; Carboniferous, Dinantian, Cumbria, UK; Â1: (H) cross section and (I) longitudinal section (J, K) Lithostrotion araneum, massive cerioid colony; Carboniferous, Dinantian, northern England, UK; Â1.3: (J) cross section and (K) longitudinal section (L, M) Actinocyathus floriformis, massive cerioid colony showing corallites with well developed axial structures; Carboniferous, Namurian, Great Limestone, Co Durham, UK; Â1.3: (L) longitudinal section and (M) cross section