140 HISTORICAL PALZAONTOLOGY. 
Devonian age; and the whole of this group has become ex- 
tinguished—unless we refer here the still surviving Deictyoneme. 
4 
Z, 
i 
Fig. 79.—a, Part of the under surface of Stromatopora tuberculata, showing the 
wrinkled basement membrane and the openings of water-canals, of the natural size; 4, 
Portion of the upper surface of the same, enlarged ; c, Vertical section ofa fragment, mag- 
nified to show the internal structure. Corniferous Limestone, Canada. (Original.) 
The Celenterates, however, are represented by a vast number 
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 
