DEVONIAN AND OLD RED PERIOD. 



143 



included corals can only be satisfactorily studied by means of 

 polished sections. In other cases, again, the corals have been 

 more or less completely converted into flint, as in the Cornifer- 



Flg. 81 . Zaphrentit cornicula, of the 

 natural size. Devonian, America. (Orig- 

 inal.) 



Fig. 80. Cystiphyllum vesiculotum, 

 showing a succession of cups produced by 

 budding from the original coral. Of the 

 natural size. Devonian, America and 

 Europe. (Original.) 



Fig. 82. HeHophyllum exiguum, view- 

 ed from in front and behind. Of the nat- 

 ural size. Devonian, Canada. (Original.) 



ous limestone of North America. When this is the case, they 

 often come, by the action of the weather, to stand out from 

 the enclosing rock in the boldest relief, exhibiting to the ob- 



