I would remind the reader that the white chalk, together with the various strata of sand, clay, and limestone, comprising the cretaceous formation of England, must be regarded as an ancient ocean-bed; in other words, an accumulation of earthy sediments, formed in the profound depths of the sea, in periods of long duration and of incalculable antiquity, and more or less consolidated by subsequent chemical and mechanical agency. These deposits are made up of organic and inorganic materials: the former consist of the debris of the cliffs and shores which encompassed the ancient ocean, of the spoils of the land brought into the waters by floods and rivers, and of mineral matter thrown down from chemical solutions. The organic substances are the durable remains of the animals and plants which lived and died in the sea, and of terrestrial and fluviatile species that were transported from islands or continents by rivers and their tributaries. The whole constitutes such an assemblage of strata as would probably be presented to observation, if a mass of the bed of the Atlantic 2,000 feet in thickness, were elevated above the waters, and became dry land; the only essential difference would be in the generic and specific characters of the imbedded animal and vegetable remains. The vestiges of terrestrial and fluviatile animals and plants found in the chalk are comparatively but few: I have collected from Kent and Sussex, bones of gigantic land lizards, (the _Iguanodon_), of flying reptiles, (_Pterodactyles_), and of fresh-water Turtles, and water-worn fragments of stems of coniferous trees allied to the _Araucaria_ or Norfolk Island Pine; fruits or aments of coniferse; and stems and foliage of plants related to the _Cycas_ and _Zamia_. [Illustration: Lign. 21:--Fragment of coniferous wood in flint.] A fragment of silicified wood imbedded in a flint, is represented in _Lign. 21_. It was obtained from a wall in Lewes Priory in Sussex; and though it has been exposed to the atmosphere seven or eight centuries, still exhibits the characteristic internal structure. Note III. Page 20. _Whitby Ammonites._ [Sidenote: AMMONITES.] [Illustration: _Lign. 22_:--Ammonites from the cretaceous formation. 1. _Ammonites varians_, from Hamsey. 2. _A. Dufresnoyi_: 2_a_, part of the same. 3. _A. lautus_: 3_a_, keel and septum of the same. ] The Ammonites differ from the Nautili in having the margins of the septa or internal shelly partitions (which in the latter are smooth), foliated or wrinkled; and the siphunculus or tube placed along the back of the shell, whereas in the Nautilus it is central. The sides of the shell in the Ammonites are very generally more or less ornamented with arched elevations and depressions, and studded with spines and tubercles, as in the specimens above figured.