In Omphalodium pisacomense and Omphalora arizonica the umbilicate thallus is limited on both sides by strongly gelatinized, prosoplectenchymatous cortices. Pseudocyphellae are formed abundantly on the top of tubercles and ridges; the tuberculate pseudocyphellae are predominantly confined to the upper surface in Omphalodium pisacomense and to the lower surface in Omphalora arizonica. Studies of the ascocarp ontogeny demonstrated the affiliation of Omphalodium and Omphalora to Parmeliaceae s. lat. In both genera, pycnidia and apothecia arise in the upper cortex, being differentiated entirely by cortical hyphae. The parmelioid development of the apothecia differs in the two genera. In Omphalodium pisacomensethe apothecium is incompletely zeorine, since the upper margin of the exciple remains covered by the thalline cortex, while in Omphalora arizonica the upper margin of the exciple protrudes and develops gradually, together with the thalline stalk, into a pedicel for the hymenium. The differences in developmental morphology support the view that Omphalodium and Omphalora should be treated as separate genera. The peculiarities of the parmelioid ontogeny and its implication in the evolution of lichens are discussed.