This paper shows the relationship between two genera of Upper Toarcian ammonites : Onychoceras and Hammatoceras. Two species have been studied : Onychoceras differens Wunstorf and Hammatoceras simulator Monestier. The Onychoceras are microconch forms and the Hammatoceras are macroconch forms. The ontogeny of Hammatoceras simulator has been established ; it is identical to that of Onychoceras. Moreover the similarities of their ornamentation, which are striking, have been established. We conclude that this is a case of sexual dimorphism and propose the attachment of the group of Onychoceras to the group of Hammatoceras.