116 THE GEOLOGICAL HISTORY OF PLANTS. 



of America and Europe the species of any of our ordinary- 

 trees, as oaks, birches, or maples, may almost be counted 

 on one's fingers, Schimper in his vegetable palaeontology 

 enumerates about eighty species of Carboniferous Sigil- 

 laricB ; and while on the one hand many of these are so 

 imperfectly known that they may be regarded as uncer- 

 tain, on the other hand many species must yet remain to 

 be discovered.* Now, in so vast a number of species 

 there must be a great range of organisation, and, indeed, 

 it has already been attempted to subdivide them into 

 several generic groups. The present state of the question 

 appears to me to be this, that in these SigillaricB we have 

 a group divisible into several forms, some of which will 

 eventually be classed with the Lepidodendra as lycopods, 

 while others will be found to be naked-seeded phseno- 

 gams, allied to the pines and cycads, and to a remarkable 

 group of trees known as Cordaites, which we must shortly 

 notice. 



Before considering other forms of Carboniferous vege- 

 tation, let us glance at the accumulation of coal, and the 

 agency of the forests of Sigillarice therein. Let us im- 

 agine, in the first instance, such trees as those represented 

 in the figures, growing thickly together over vast swampy 

 flats, with quantities of undergrowth of ferns and other 

 plants beneath their shade, and accumulating from age to 

 age in a moist soil and climate a vast thickness of vege- 

 table mould and trunks of trees, and spores and spore- 

 cases, and we have the conditions necessary for the growth 

 of coal. Many years ago it was observed by Sir William 

 Logan that in the coal-field of South Wales it was the 

 rule with rare exceptions that, under every bed of coal, 

 there is a bed of clay filled with roots of the Stigmaria, 

 already referred to as the root of Sigillaria. This dis- 



* In n recent memoir (Berlin, IBS'?) Stur has raised the number of 

 species in one subdivision of the SigUlance (the Favularia) to forty- 

 seven t 



