Pleospongia (Archaeocyathi) are an independent class of Porifera. Stratigraphic range, limited to Lower and Middle Cambrian, is remarkable in the light of worldwide distribution in Australia, North America, Europe, Africa, and Asia. Some 400 species are known, about 35 on this continent. The most typical are goblet-shaped with distinct inner and outer walls, connected by parieties, tabulae, or irregular vesicular tissue. Numerous pores give the skeleton an appearance of exceedingly fine mesh. Geographic and stratigraphic distribution of Pleospongia, ecology, and faunal associations are considered. Morphology, ontogeny, and phylogeny are described in detail. Five subclasses—Monocyatha, Archaeocyatha, Anthocyatha, Exocyatha and Uranocyatha—show fundamental differences; their relationship is indicated by early stages and methods of skeletal construction. Monocyatha have single walls, laminar, perforated by regular pores, or more or less spongy. Archaeocyatha contain all types invariably having double-walled skeleton, cup, beaker, or saucershaped. The spitzes strongly resemble the Monocyatha, suggesting derivation from Monocyathus-like ancestors. Four orders are Acanthinocyathina Okulitch, Syringocnemina Okulitch, Ajacicyathina Bedford, and Metacyathina Bedford. Anthocyatha superficially resemble the Anthozoa. Uranocyatha include hollow spherical, oval, or gastrula-like shapes. Exocyatha are independent or encrusting Pleospongia consisting of irregular vesicular or cellular tissue without definite walls. In Labrador, Quebec, New York, Virginia, Georgia, California, Nevada, Washington, and British Columbia Pleospongia are confined to the Lower Cambrian. A single doubtful species occurs in the Middle Cambrian of New Brunswick. Practically all built extensive reefs. All known American species are fully described and illustrated. Since 1935 the number of known American forms has increased from 13 to 35. Their importance as index fossils has greatly increased. Pleospongia, hitherto neglected and very imperfectly known, represent one of the most important groups of Cambrian organisms.