In mammals the fibers of the dorsal lateral olfactory tract either pass under the accessory olfactory formation, or they penetrate through it separating the internal granule cells from the output cells. The use of this trait as a phylogenetic indicator in 181 specimens representing 131 species of 16 orders yielded evidence for common ancestry of Insectivora, Chiroptera, Dermoptera, Rodentia, and Primates (including Tupaia), since all share the derived trait, their dorsal lateral olfactory tract fibers passing through the accessory olfactory formation. Carnivora (including Pinnipedia), Hyracoidea, Perissodactyla, and most Artiodactyla share the primitive condition (fibers passing under) with the one order of monotremes and three marsupial orders. The Edentata and Lagomorpha may be separate from the two major placental groups and from each other, or they may represent successive stages in the evolution of the derived state through progressive alterations in the relative chronology of development of olfactory system components, or one or both orders may occupy an ancestral position with respect to the dichotomy within placental mammals.