The known Ctenostomatous Bryozoa and similar, but not necessarily related, adnate and penetrating forms in Carboniferous and Permian marine shells of America are described and discussed. Only the genera Ascodictyon, Vinella, and Allonema, and most species of Rhopalonaria and Heteronema are thought to be true ctenostomes. The principles on which distinction between Ascodictyon and Vinella is based are revised and redefined. Affinity of Bascomella, a supposed ctenostome, is considered doubtful, while Plychocladia is found to be not a bryozoan but a Foraminifera and is described in a separate paper. The first American representative of the rare European genus Caulostrepsis, a pseudoboring possibly produced by a worm, is described and discussed. Five new species of ctenostomes and seven new species of doubtful affinity are described.