Page:Early Man in Britain and His Place in the Tertiary Period.djvu/38

10

The Tertiary or Kainozoic strata were divided by Sir Charles Lyell in 1833, into three great groups, according to the percentage of existing mollusca, which was presented in a comparison of 3000 fossil with 5000 living forms. The Eocene ( dawn, new), or the earliest group, contained about 3$1⁄2$ per cent of living shells, and thus, to speak metaphorically, was characterised by the dawn of the Testaceous fauna, now living in the sea. In the Meiocene ( less, ) group the existing forms were much more abundant, being always less than 35 per cent. The upper group was termed Pleiocene ( more, ), because it presented from 35 to 50 and even 90 per cent of living testacea. The vast number of fossil species which have since been added to those which formed the basis of this classification has not materially altered its value, but merely rendered the strict definition of the percentages impossible. The term Pleistocene ( most, ) was subsequently applied by Sir Charles Lyell to assemblages of fossil species in which there was a still nearer approximation to existing nature.