414 FOSSIL INVERTEBRATES/Porifera Figure Representative calcareous and ‘sphinctozoan’ sponges (A) Stellispongia, Permian, Tunisia, an inozoan calcar eous sponge (B) Cystothalamia, ‘sphinctozoan’ hypercalcified demosponge, Permian, Texas, USA (C) Girtyocoelia, ‘sphintozoan’ hypercalcified demosponge, Permian, Texas, USA All are natural size Adapted with permission from Rigby JK (1987) Phylum Por ifera In: Boardman RS, Cheetham AH, and Rowell AJ (eds.) Fossil Invertebrates Palo Alto, Oxford, London: Blackwell Scientific Pub lications Figure Diagrams of enlarged hexactinellid skeletal struc tures (A) Lyssacid grade reticulate skeletal structure with unfused or weakly fused overlapping hexactine spicule rays (B) Dictyid or hexactinosan grade with overlapping spicule rays fused into a rigid, three dimensional, quadrangular skeletal meshwork (C) Lychniscoid grade with overlapping rays fused into a rigid skeletal meshwork and with diagnostic octahedral ‘lanterns’ at the centre of each spicule workers to also include the taxonomically limited fossil chaetetids and the diverse stromatoporoids Stromatoporoids had long been considered as coelenterates related to hydrozoans, but recently they have been moved to the Porifera These sponges have a laminated calcareous skeleton that is perforated by astrorhizal canals, and the living animal is thought to have occupied essentially only the upper surface If the Stromatoporoidea are included, the class Sclerospongiae has a stratigraphical range from the Cambrian to the Holocene, with a major period of development in the Silurian and Devonian, when these organisms were important reef formers The class became a minor marine element in the Late Palaeozoic, however, and remains so in modern marine faunas Stromatoporoids have domal, tabular, branching, or bulbous growth forms They are commonly laminated to vertically tubular or pillared, and have generally aspiculate, calcareous skeletons in early forms and more cystose and vertically tubular skeletons in later forms Laminate forms commonly show cyclicity, or growth interruptions, with thicker latilaminae separated by several thinner sheet-like porous laminae (Figure 9) These laminae are interconnected or supported by vertical pillars that may extend only between adjacent laminae or may be superimposed through several laminae They range from long and