Structural features indicative of a semi-internal or internal position for the shells are reviewed in representatives of three Cretaceous genera: Gaudryceras and Ptychoceras, both belonging to Lytoceratida, and Aconeceras which is a member of Ammonitida. In Gaudryceras, the shells are covered by thick coating layers with lamellar structure, similar to that of the nacreous layer. Very probably these layers were secreted by the mantle epithelium which covered the shell surface. In Ptychoceras, the semi-internal or internal position of the shell is indicated by the following features: (1) the mode of truncation of the initial portion of the shell in three species; (2) the frequent alterations of the ultrastructure of the shell wall during ontogeny; and (3) the formation of a thin coating layer with nacreous strncture on the shell surface. Also, in Aconeceras, the shell surface was quite probably covered by the mantle, at least at the apertural region. This is indicated by: (1) the secretion of an additional porous spherulitic layer on the outer surface of the keel; (2) the formation of a thin coating layer on the lateral sides of fully grown shells; and (3) the structural changes of the shell wall during ontogeny.