The taxonomie implications of dimorphism are once again discussed. The author concludes that a classification in which the microconch groups are considered as subgenera of the macroconch genera implies the existence of multiple subordinations : 1 microconch subgenus corresponding to 2 or more macroconch genera. Microconch taxa are therefore classified at a generic level. On the other hand, the author presents the results of a study on the ontogeny and biometry of the genera Zugodactylites, Catacoeloceras, Porpoceras and Nodicoeloceras whose microconch homologues are respectively the genera Gabillytes, Mucrodactylites and Collina for the last two. The phylogeny of the Dactylioeeratidae is briefly discussed.