In the Cretaceous deposits of the southern sea, where the Ammonite tribe is almost unknown, the remarkable family of the lamellibranchs known as the Hippuritidæ furnish the dominant invertebrates of the period, and the representatives of this family are exceedingly scarce amongst the Cretaceous strata of the northern gulf, though they are found on two or three horizons. Of vertebrates, the most interesting are the reptiles. The families which predominate in Jurassic times have many representatives amongst the Cretaceous strata also, but the order Squamata is represented by the sub-order Pythonomorpha, which is characteristic of the Cretaceous rocks. The best known representative is the gigantic _Mosasaurus_. Lastly, we have the remarkable toothed birds or Odontornithes, now placed in different orders, the genus _Hesperornis_ being the only representative of the sub-order Odontolcæ of the Ratitæ, whilst _Ichthyornis_ and allied forms are placed in the sub-order Odontormæ of the Carinatæ. CHAPTER XXIV. THE EOCENE ROCKS. _Classification._ The Eocene Beds of the south of England have been subdivided according to the variations in their lithological characters, and the subdivisions have received local names. The following classification is generally adopted, though the different subdivisions are by no means of equal value: Upper Eocene { Upper Bagshot Beds { Barton Beds Middle Eocene Bracklesham Beds { Lower Bagshot Beds { London Clay[103] Lower Eocene { Oldhaven Beds } Lower London { Woolwich and Reading Beds } Tertiary Strata { Thanet Sands } [Footnote 103: Some writers place the London Clay in the Middle Eocene.] The deposits vary greatly when traced abroad, and the exact equivalents of the minor subdivisions of the British rocks can seldom be ascertained at any distance from England, though the division into Upper, Middle, and Lower Eocene can be made over wide areas.