The rare ambocoeliinid genus Metaplasia Hall & Clarke, 1893, occurs only in North America. A new ambocoeliinid genus, Plicoplasia, occurs in North America, South America, and South Africa. Plicoplasia is notable for the development of strong radial plications, a feature absent in most genera belonging to the Ambocoeliinae. Metaplasia is closely related to Ambocoelia, but the presence of a low fold and low lateral plications on the brachial valve of the former serves to distinguish the two genera. Both Metaplasia and Plicoplasia are known from strata of Siegen to Ems age (=Oriskany to Onondaga age), and could conceivably have been derived from the earliest Lower Devonian species of Ambocoelia described by Kozlowski (1929) from Podolia. Ambocoelia is also known in the late Lower Devonian of Czechoslovakia and Bolivia. A spinoise, weakly plicate new ambocoeliinid genus, Spinoplasia, has been recognized in strata of Gedinnian (=New Scotland) age in Gaspe.