The ligaments and folds of the middle ear are compared with the mesentery of the abdominal cavity. From a philogenetic point of view, these structures first appear in the cynodonts. These ligaments maintain the malleus and the incus in the tympanic cavity, increasing the transmission of the auditory system. Some of these ligaments and folds are different in some mammals. A mucous fold has been described in the posterosuperior quadrant of the human tympanic membrane. It emerges from the highest portion of the handle of the malleus near its neck and runs back and up to descend in the ceiling of the tympanic cavity. In this paper we have searched this mucous fold in 104 ears of 12 mammalian species, including three species of primates. We found it only in 4 Ateles monkeys, where it is similar to that in man. We think that this fold is a vestigial embryonic structure. The knowledge of these folds is important for advances in aural microsurgery; and to know more about the structures that have phylogenetic relations in the evolution of the anatomy of the ear in the animal kingdom.