The epoch at which these important advances were made, and the foundation of the Mammal class was laid, may be put with great probability in the first section of the Mesozoic or secondary age--the Triassic period. The oldest fossil remains of mammals that we know were found in strata that belong to the earliest Triassic period--the upper Kueper. One of the earliest forms is the genus Dromatherium, from the North American Triassic (Figure 2.268). Their teeth still strikingly recall those of the Pelycosauria. Hence we may assume that this small and probably insectivorous mammal belonged to the stem-group of the Promammals. We do not find any positive trace of the third and most advanced division of the Mammals--the Placentals. These (including man) are much younger, and we do not find indisputable fossil remains of them until the Cenozoic age, or the Tertiary period. This paleontological fact is very important, because it fully harmonises with the evolutionary succession of the Mammal orders that is deduced from their comparative anatomy and ontogeny. The latter science teaches us that the whole Mammal class divides into three main groups or sub-classes, which correspond to three successive phylogenetic stages. These three stages, which also represent three important stages in our human genealogy, were first distinguished in 1816 by the eminent French zoologist, Blainville, and received the names of Ornithodelphia, Didelphia, and Monodelphia, according to the construction of the female organs (delphys = uterus or womb). Huxley afterwards gave them the names of Prototheria, Metatheria, and Epitheria. But the three sub-classes differ so widely from each other, not only in the construction of the sexual organs, but in many other respects also, that we may confidently draw up the following important phylogenetic thesis: The Monodelphia or Placentals descend from the Didelphia or Marsupials; and the latter, in turn, are descended from the Monotremes or Ornithodelphia.