Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 Perna MulUU. Desh. and Leym. a. Exterior. b. Part of hinge of upper valve. preexisting rocks, from the wearing, down of which the Greensand beds were derived. The land, consisting of such rocks, was doubt- less submerged before the origin of the white chalk, a deposit which originated in a more open sea, and in clearer waters. The fossils of the Lower Cretaceous are for the most part specific- ally distinct from tho
Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 Perna MulUU. Desh. and Leym. a. Exterior. b. Part of hinge of upper valve. preexisting rocks, from the wearing, down of which the Greensand beds were derived. The land, consisting of such rocks, was doubt- less submerged before the origin of the white chalk, a deposit which originated in a more open sea, and in clearer waters. The fossils of the Lower Cretaceous are for the most part specific- ally distinct from those of the Upper Cretaceous strata. Among the former we often meet with the genus Scaphites or Ancyloceras (fig. 331), which has been aptly described as an ammo- Fig. 331 j Ancyloceras gigas, D'Orb. Nautilus plicatw, Sow., in Fitton's Monog. nite more or less uncoiled; also a farrowed Nautilus, JV. plicatus (fig. 332), Trigonia caudata (fig. 334), likewise found in the Black- down beds (sec above, p. 332), and Gervillia, a bivalve genus allied to
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