The morphology of the tentorium cerebelli was investigated in representatives of the chordata. A tentorium cerebelli characterized birds and mammals but was regularly absent in fishes, amphibians, and reptiles. The data suggest that the tentorium cerebelli emerged relatively late in phylogeny as bilateral folds of dura mater on either side of the brainstem in the cerebro-cerebellar fissure. Subsequent evolution was characterized by the union of these septa behind the brainstem and by a progressive lengthening of the straight sinus. Tentorial ossification distinguished a further evolutionary step in some mammalian lines. The inferred evolutionary pattern of the tentorium cerebelli resembles the successive stages which the human tentorium undergoes during ontogeny.