four ancestral groups of the four main orders of Placentalia—namely, of the Primates, Ungulata, Rodentia, and Carnassia. They seem to be so closely related by their primitive organization that they may be united in one common super-order, Prochoriata.
With a considerable degree of probability, we are led to formulate the further hypothesis that all the orders of Placentalia—from the lowest Prochoriata upwards to man—have descended from some unknown common ancestor living in the Cretaceous period, and that this oldest placental form originated from some Jurassic group of marsupials.
Among these numerous fossil Lemures which have been discovered within the last twenty years, there exist, indeed, all the connecting forms of the older series of Primates, all the 'missing links' sought for by comparative odontology.
The oldest Lemures of the tertiary age are the Eocene Pachylemures, or Hyopso-
