A monographic study of the catapyrenioid lichen genera in North America, from arctic Canada to temperate Mexico, is presented in a series of four papers. Catapyrenioid genera are characterized by a squamulose or squamulose-areolate thallus, perithecia lacking interascal filaments and hymenial algae, and simple or one-septate ascospores. Altogether 36 species belonging to seven genera are treated: Catapyrenium (4 spp.), Clavascidium (3 spp.), Heteroplacidium (5 spp.), Involucropyrenium (1 sp.), Neocatapyrenium (2 spp.), Placidiopsis (4 spp.), and Placidium (17 spp.). Keys to genera and species are presented and the circumscriptions of the genera are discussed. For each species a detailed description and accounts on ecology and distribution are provided. Distribution dot maps of more common species are supplied. Key words: Lichenized Ascomycota, Verrucariaceae, taxonomy, mycobiota of Canada, United States, Mexico.