The vertebrate fauna is very noteworthy. The fishes and reptiles are closely related to existing forms, and the orders of reptiles which predominated in Mesozoic times have completely disappeared. But the mammals are the most interesting vertebrates of the Eocene period. Instead of the lowly organised forms of Mesozoic times, we find representatives of many orders, including the highest, the Primates. The generalised forms which serve as links between groups which are now separated to a considerable extent are of particular importance. They have been detected in Eocene rocks of various regions, though the most complete series have been obtained from the Eocene rocks of North America and made known to us through the numerous memoirs of Professors Cope and Marsh[105]. [Footnote 105: The Eocene floras of Britain are described by Mr J. Starkie Gardner and Baron von Ettingshausen in the _Monographs of the Palæontographical Society_; other Monographs of the same Society contain an account of the Eocene Mollusca by Mr F. E. Edwards and Mr S. V. Wood. An idea of the generalised forms of Mammalia may be obtained by perusal of that portion of Nicholson and Lydekker's _Manual of Palæontology_ in which the latter author treats of the Mammalia, and in this connexion the reader will do well to read Prof. Huxley's "Lecture on Fossil Horses," reprinted in his _American Addresses_.] CHAPTER XXV. THE OLIGOCENE AND MIOCENE PERIODS. (i) _The Oligocene Beds._ _Classification._ The Oligocene Beds of Britain are classified as follows:-- Upper Wanting Middle Hempstead Beds { Bembridge Beds Lower { Osborne Beds { Headon Beds _Description of the strata._ Little need be said of the deposits of this period, either in Britain or abroad, except to remark that they show the further spread of continental conditions over the regions now occupied by land. The British deposits are now seen in the Hampshire Basin only, and have been spoken of as the fluvio-marine series, as many of the strata were laid down in continental sheets of water, while the true marine sediments are thin and infrequent. The lithological characters of deposits formed under these conditions naturally vary greatly, consisting of different kinds of mechanical sediments occasionally mixed with thin freshwater marls and limestones. On the Continent similar conditions prevailed, though the occurrence of fairly wide tracts of level surface is indicated by the widespread distribution of beds of brown coal or lignite, and the coarse and thick Oligocene 'nagelfluh' of Switzerland points to the elevation of mountain ranges in the neighbourhood. _The flora and fauna._ The remarks made concerning the Eocene flora and fauna are generally applicable to those of Oligocene times, except that the Oligocene fossils bear a still closer resemblance to living forms, and the Nummulites are no longer dominant.