io: 



THE STORY OF THE EARTH. 



Such univalve or Gasteropod shells as Chiton, 

 Littorina % Natica, Patella, Pkurotomaria % Turritella 



and Turbo are surviving genera which are found 

 in the Carboniferous Limestone. These are not 



ays the genera richest in species. Aviculo- 



pecten, which is regarded as extinct, has more 



species than any other bivalve ; and EuomfJialus, 



extinct, is one of the best represented genera 



of gasteropods. 



Fishes abound, the cartilaginous fishes, or 

 sharks, appear curiously to parallel both in their 

 fin defences and teeth, the sharks which are sub- 

 sequently found in the Secondary rocks. The 

 ganoid fishes are also well represented. As a 

 group they are unlike the types which lived in 

 the old red sandstone time. 



Millstone Grit. 



This rock, mainly formed of quartz grains, is 

 a shallow-water deposit, which often gives evi- 



dence of current bed- 

 ding, and sometimes di- 

 vides along planes in 

 which the mineral mica 

 is abundantly deposited. 



There is perhaps 

 good ground for separa- 

 ting these locks from 

 the overlying coal 



inal measures ; and the sand- 



" £■ «* stones which they con- 



Ciilx miferous limestone. 



tain are not more im- 

 portant than those known as the Pennant sand- 

 stones, in tl measures of the Wesl of I 



laud. i lures Of the 



I \> .. i J.- Pletin >t<-niari.i. ihow- 



