Volume 4 covers hypercalcified sponges. After a chapter on living members, the treatise then focuses on fossil hypercalcified chaetetid-type Porifera (Demospongiae), treating their functional morphology, classification and evolution, palaeoecology, and palaeogeography and biostratigraphy. It also introduces post-Devonian hypercalcified sponges of the Stromatoporoid type before giving systematic descriptions of both groups. This is followed by a lengthy section describing all known classes, orders, and families.
Volume 5 covers the palaeozoic Stromatoporoidea, describing external and internal morphology, microstructure and mineralogy, morphologic affinities to other fossil and recent groups, functional morphology of the skeleton, early evolution, diversity trends, extinction patterns, biostratigraphy, palaeoecology, palaeobiogeography, techniques of study, and classification. This is followed by a lengthy section describing all known classes, orders, and families.