Fifteen characters of brain organization are identified for which primitive and derived states can be distinguished, and which are thus useful in reconstructing mammalian phylogeny. The states of the characters are numerically coded in sequence from most primitive to most derived, to facilitate reciprocal comparisons in comprehensive genealogical analyses. The characters include certain features of cerebral circulation, cytoarchitecture, fiber pathways, and sensory projections. Seven characters are shared with nonmammals, a fact which makes them particularly useful for establishing their primitive vs. derived states; the other eight characters are features peculiar to mammals. Ten characters provide impressive reinforcement for traditional major groupings of the species, while five provide bases for new ideas about mammalian relationships.