The most influential early classification of the group appeared in H.A. Nicholson's great monograph on the stromatoporoids for the Palaeontographical Society (1886–1892). He believed that the stromatoporoids were similar to the two major groups of living hydrozoans and based his major divisions, the Hydractinoidea and the Milleporoidea, on this similarity. He divided the first of the two groups into the families Actinostromatidae (distinct laminae and pillars) and Idiostromatidae (dendritic forms), and the second group into the Stromatoporidae (amalgamate structures of coenosteles and coenostroms) and the Labechiidae (horizontal elements largely dissepiments). These basic divisions have continued to be recognized in later classifications but additional groups have been split off from Nicholson's original four and his bases of classification have been changed. Lecompte (1956) in the Treatise volume recognized, in addition, the Clathrodictyidae for genera with laminar subvesicular structure and compact tissue; the Stromatoporellidae for genera with dominantly laminar structure and porous tissue, and the Syringostromatidae for genera with well differentiated laminae and pillars and “vertically tabulate chambers”. He also continued a practice started by Dehorne (1920) of including Mesozoic stromatoporoids in Paleozoic families. He recognized the families Milleporellidae and Milleporidiidae for exclusively Mesozoic stromatoporoids. Most later classifications have ignored the Mesozoic stromatoporoids and grouped only Paleozoic genera. Recent works on the classification of Paleozoic stromatoporoids include those of Galloway (1957), Bogoyavlenskaya (1969), Khalfina and Yavorsky (1973), and Stearn (1980).