The names in the first column indicate the primary divisions or life-periods, while those in the second column are the great systems, each of which is again divided into minor groups, the popular names of a few of these minor groups being given in the third column. There are at present no means of arriving at any satisfactory conclusion as to the absolute length of time indicated by either the primary or secondary divisions; but there is little doubt that the whole of the Tertiary period is only equal to a fraction of the Mesozoic as regards its duration, while it is probable that the duration of the Mesozoic epoch was largely exceeded by that of the Palæozoic. _Mesozoic Mammals._—The earliest date at which mammals are at present known is in the upper part of the Triassic period, which forms the base of the great Mesozoic epoch; and from this date they are represented more or less abundantly in various horizons of the Jurassic and Cretaceous. The very rapid advances in our knowledge of these forms which have been made in the last few years, especially in consequence of the explorations of rich fossiliferous beds in North America, have not only completely changed the present aspect of the science, but give such promise for the future, that any sketch which we may now attempt of this branch of the subject can only be regarded as representing a transient phase of knowledge. It will be well, however, to gather together in this place the leading facts now ascertained with regard to the most ancient forms, as, owing to the uncertainty of their relationship with any of the existing orders, they will be most conveniently treated of separately, while the ascertained facts relating to the geological history of the forms more nearly allied to those now living will be more appropriately described under the account of the different groups into which the class may now be divided.