A review of the olfactory formation in over 100 mammals revealed that in some species the dorsal component of the lateral olfactory tract (dLOT) runs above the internal granule (IG) cells of the accessory olfactory formation, while in others the tract runs below the IG cells. The initial analysis of grouping the mammals according to the position of the dLOT demonstrated a division among the placental mammals that correspond to that based on fossil evidence. Recently, specimens from three orders of placentals (elephant, shrew, bat and pika) became available which had not yet been examined for this feature, thus affording a more complete data base. In all of these new species the dLOT passes through the accessory olfactory formation; thus they are to be grouped with rodents, primates and insectivores. Beyond the value of taxonomy, these data permit the generation of hypotheses concerning the temporal dynamics of ontogeny.