CARBONIFEROUS. 97 



embryonic. They have great importance in the 

 primary period. their living representa- 



tives r, has, in addition to the gills 



which are common to most fishes, a lung which is 



ipted to breathing air under terrestrial condi- 

 tion-, a> though it ss for some fishes 

 ive developed terrestrial habits of life. 



Another interesting circumstance COnne 

 with the old red sands currence in 



it, in the [rish locality of Kiltorkan, and 1. 



:i in Monmouth, of a shell, which has not 



been distinguished from the common pond muss 



named An 



:i ( Vnada make known, 



among evident stria! life, large inset 



and remark rms of myriapods, in which 



there IS Only I me >ed on each 



the body. 



The accumulation of the enormous thicknes 



of the old red sandstone deposits presupposes im- 

 mense dim< for a lake in which sediments 

 thre< ould be piled up, and a la 

 a oi denudation to furnish it with sediments. 



CHAPTER XIII. 



CARBONIFEROUS. 



The carboniferous period of time has been so 

 named because it is the principal geological epoch 

 in Britain in which coal occurs. The rocks rest 

 on the Old Red Sandstone in Scotland, and on the 

 Devonian in the west of England. They are un- 

 conformable to the older rocks in some districts. 



