140 HISTORICAL PALEONTOLOGY. 



Devonian age ; and the whole of this group has become ex- 

 tinguished unless \ve refer here the still surviving Dictyonemx. 



Fig. 79. a, Part of the under surface of Stromatopora. tubercnlata, showing the 

 wrinkled basement membrane and the openings of water-canals, of the natural size; b, 

 Portion of the upper surface of the same, enlarged; c, Vertical section of a fragment, mag- 

 nified to show the internal structure. Corniferous Limestone, Canada. (Original.) 



The Ccelenterates, however, are represented by a vast nu,mber 

 of Corals, of beautiful forms and very varied types. The 

 marbles of Devonshire, the Devonian limestones of the Eifel 

 and of France, and the calcareous strata of the Corniferous 

 and Hamilton groups of America, are often replete with the 

 skeletons of these organisms so much so as to sometimes 

 entitle the rock to be considered as representing an ancient 

 coral-reef. In some instances the Corals have preserved their 

 primitive calcareous composition ; and if they are embedded 

 in soft shales, they may weather out of the rock in almost all 

 their original perfection. In other cases, as in the marbles of 

 Devonshire, the matrix is so compact and crystalline that the" 

 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- 

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

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



