The tetradiids were the most common tabulate coral component of the Ordovician (Caradocian–Ashgillian) sequence in the Manitoulin District, locally abundant enough to form biostromal accumulations. Five species, assigned to four genera, including a new subgenus Paenetetradium, are recognized in the section. In sequence, from oldest to youngest, these species are: Tetradium (Tetradium) undulatum n.sp. from the Cloche Island Formation; Rhabdoietradiitm giganteum n.sp., Paratetradium capella n.sp., and Phytopsis cf. racemosum (Raymond) from the Cobourg Formation; and T. (Paenetetradium) huronensis (Billings) n.subgen. from the Meaford and Kaguwong Formations, Broad trends include the development of tabulale from atabulate forms, reduction in corallite sizes, and dominance of cerioid forms by the late Ordovician. Tetradiids were commonly overgrown by stromatoporoids, calcareous algae, or bryozoans, and, in biostrome concentrations, grew in very shallow waters.