188 WHAT IS LIFE ? 



The species which existed in Silurian times are 

 largely replaced by other species. 



Plant life, not bearing flowers, already existed when 

 these layers were deposited. Ferns and huge tree-like 

 club-mosses then lived, as also cone-bearing trees, like 

 the firs. 



The remains of animal life in these formations are 

 both freshwater and marine. Corals are abundant, 

 but the prevalent species are different from those of 

 the Silurian system. The crinoids sometimes formed 

 entire beds of rock. The lower classes of shell-fish 

 are still abundant. Trilobites, which were hitherto 

 abundant, begin to be scarce. Insects and scorpions 

 lived when these formations were being deposited. 

 But these were times when fishes began to prevail 

 over the large Crustacea and their allies. The ganoids 

 fishes similar in type to our sturgeons were the 

 most varied in these waters. One species was most 

 formidable, specimens having been found where the 

 head alone, enclosed in strong plates, attained a 

 length of three feet, and was armed with a powerful 

 apparatus of teeth. 



The Devonian rocks are found in Britain, Norway, 

 Central Europe, Russia, North America, and Australia. 



The Carboniferous strata are those which were 

 deposited over and upon the Devonian rocks. The 

 system obtains its name from the numerous seams of 

 coal which exist in it. Again there is no sharp line 

 between these layers and the previous division. It 

 must not be thought that these strata consist solely of 

 coal, this is only a small part of the formation. Coal 

 exists in thin as well as thick layers. A seam may 

 consist of cannel coal at one part of a mineral field, and 



