142 



HISTORICAL PALAEONTOLOGY. 



pora also exhibit on their surface the rounded openings of 

 canals, which can hardly have served any other purpose than 

 that of permitting the sea-water to gain ready access to every 

 part of the organism. 



No true Graptolites have ever been detected in strata of 

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

 tinguished unless we refer here the still surviving Dictyonemce. 

 The Ccelenterates, 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 



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 ; 

 6, Portion of the upper surf ace of the same, enlarged; e, Vertical section of a fragment, 

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



and of ^rance, 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 



