508 DICOTYLEDONOUS PLANTS. 
The number of species of plants in the various 
divisions of the Tertiary strata, is as yet imper- 
fectly known. In 1828, M. Ad. Brongniart con- 
sidered the number then discovered, but not all 
described, to be 166. Many of these belonging 
to Genera at that time not determined. The 
most striking difference between the vegetables 
of this and of the preceding periods is the abun- 
dance in the Tertiary series, of existing forms of 
Dicotyledonous Plants and large trees, e. g. 
Poplars, Willows, Elms, Chestnuts, Sycamores, 
and many other Genera whose living species are 
familiar to us. 
Some of the most remarkable accumulations 
of this vegetation are those, which form extensive 
beds of Lignite and Broivn-coal.^ In some parts 
of Germany this Brown-coal occurs in strata of 
more than thirty feet in thickness, chiefly com- 
posed of trees which have been drifted, appa- 
rently by fresh water, from their place of growth, 
and spread forth in beds, usually alternating with 
sand and clay, at the bottom of then existing 
lakes or estuaries. 
The Lignite, or beds of imperfect and stinking 
Coal near Poole in Dorset, Bovey in Devon, and 
Soissons in France, have been referred to the 
first, or Eocene period of the Tertiary formations. 
* See an admirable article on Lignites by Alexandre Brongniart 
in the 26th vol. of the Dictionnaire des Sciences Naturelles. 
