The striking feature of life development in the Cenozoic era is the great progress and expansion over the earth of the mammalian races. The division of the era into periods, however, was based largely on a study of fossil mollusks. In the Paris basin of France, it was noticed by the geologists of a century ago that the youngest of the sedimentary beds contained the greatest number of recent or still living species. Successively downward into the older beds the percentage of recent species decreased until there were practically no living species represented in the oldest rocks of the series. From the percentage of recent forms among prehistoric ones it was proposed that the following division be made: Eocene, meaning _dawn of the recent_; Miocene, meaning less recent; and Pliocene, meaning _more recent_. Sometime later it was suggested that another period be added, and to this was given the name Pleistocene, meaning _most recent_. In 1854, the older Miocene formations were segregated and referred to a newly provided Oligocene period, this name meaning _little of the recent_. Early geologists grouped the rocks in three great divisions, applying the names Primary, Secondary, and Tertiary. To these was added afterwards the name Quaternary, which applied to the youngest formations of the earth. Only two of these terms remain in common use at present: it is a frequent practice to refer to the combined Eocene, Oligocene, Miocene and Pliocene periods as the “Tertiary” division of Cenozoic time; to the Pleistocene and Recent periods as the “Quaternary” division. The geology of some remote future may be clearer with regard to the full significance of this subdivision of the Age of Mammals into two parts. It may be that a great era was concluded at the end of Pliocene time as others have been concluded, by the usual earth disturbances and climatic changes and by the decline of animals once prominent in the faunas of the world. Events of such character have registered their occurrence but may eventually prove to have been a series of minor events not comparable with the revolutionary changes that terminated other great time divisions. The favored practice of including ourselves and our times in the Cenozoic is based on a trend of opinion which holds that no great era has been ended since the Age of Reptiles was concluded. [Illustration: Skulls of the clumsy, six-horned uintathere and the early, hornless titanothere form part of this Eocene display. In the mural these animals and the little “three-toed” Eohippus, smallest of horses, are pictured with a contemporary turtle and crocodile.]