We have alluded to the undulating character of the downs, so “well known,” as Mantell says, that “local details are unnecessary.” How correct this is may be seen in the following drawing, which represents a portion of Royston Heath. [Illustration: Fossil] All over this heath is found the “Royston crow,” during the winter months. This fine bird migrates hither from Norway, to avoid its severe winters, and is scientifically known as the “Hooded-crow, _corvus cornix_.” On its first arrival, when it is in its best plumage, it is comparatively tame, allowing the sportsman to approach very near; but as the season advances, acquaintance with the gun makes it very knowing and shy. It associates freely with the other crows, but its nest has never yet been found in England. About March the hooded-crow wholly disappears. The head, throat, and wings are black; the back and breast a “clear smoke-grey.” Norman, the bird-stuffer of this town, has always several fine specimens on hand. As, in the case of the Carboniferous system, we ventured to say to the reader that it was not all _coal_, so in the Cretaceous system, we would remind him that it is not all _chalk_; but without going minutely into the subdivisions which the chalk formation has received, because this unpretending elementary treatise does not profess to _teach_ geology, but simply aims, as we have ventured again and again to repeat, to infuse into the mind a desire of acquaintance with the marvels and truths of this science, we will just indicate the leading divisions and nomenclature of this deposit. First, there is the green sand; that is, _first_, beginning at the bottom or lower part of the formation: this may be well seen and studied in the neighbourhood of Cambridge, where we have procured many of its characteristic fossils, including several vertebræ and teeth of the otodus, a fish allied to the shark family, such as are figured in the opposite diagram. [Illustration: FOSSIL TEETH OF FISHES: FROM UPPER GREEN SAND, CAMBRIDGE. 1. OTODUS. 2. CARCHARIAS. 3. CORAX. 4. OXYRHINA. 5. NOTIDANUS. 6. LAMNA. 7. PTYCHODUS. ] [Illustration: FOSSILS FROM THE GAULT, FOLKSTONE. 1. AMMONITE DENTATUS. 2. AM. LAUTUS. 3. AM. SPLENDENS. 4. AM. CRISTATUS. 5. AM. DENARIUS. 6. CATILLUS SULCATUS. ]