The presence of spicule pseudomorphs and an aquiferous filtration system in Mesozoic stromatoporoids confirms poriferan affinity for this previously problematic group. The form, and postulated original siliceous nature of most of the spicules, indicates that the possessors of these were calcified demosponges; others with originally calcareous spicules were calcareans. Different spicule types and arrangements indicate that the possession of a calcareous skeleton is a convergent feature. Previous defining characteristics are found to be invalid and Mesozoic stromatoporoids are redefined as calcified sponges, so that the term 'stromatoporoid' now only refers to a grade of organisation of the calcareous skeleton and not a taxonomic grouping. 'Chaetetids', 'sphinctozoans' and 'sclerosponges' are also polyphyletic groupings, representing grades, where some members of which are calcified demosponges. These groupings should no longer be considered in isolation, as this has previously obscured their true nature. The original mineralogy of late Mesozoic stromatoporoids was probably low-Mg. calcite. Spicule and calcareous skeleton diagenetic lineages are given to enable workers to determine original microstructures. Analogies with Recent calcified demosponges, suggests that the fossil stromatoporoid demosponges produced a primary framework of siliceous spicules bound together with an organic matrix, which probably provided the nucleation sites for subsequent precipitation of the calcareous skeleton. A new taxonomic scheme is presented based on spicule type, arrangement and relationship of this framework to the microstructure of the calcareous skeleton. On the basis of spicule criteria, calcareous microstructure alone is found to be convergent and can no longer be used as a high-level taxonomic feature. Spiculate species are therefore redescribed and placed within the Recent poriferan classification framework. Synonyms are documented and intra-specific variation discussed. The present cnidarian-based nomenclature is replaced by one modified from Recent poriferan terminology. The possession of a calcareous skeleton is probably a relict feature of a previously widespread calcified sponge fauna. It appears that the calcareous skeleton was acquired independently in a number of lineages in the early Palaeozoic, and has subsequently been lost in many.