66 OUR COMMON BRITISH FOSSILS. 



belonging to the Rugosa, such as Strombodes, Cya- 

 thophylluni (perhaps the most plentiful of them all), 

 Cyathaxinia (a simple coral), Lithostrotion, Lit ho- 

 dendron, etc. Perhaps the single coral which may 

 be regarded as the simplest in structure is Am- 



Fig. 49. Recent Arborescent Perforate Coral (Ocuh'na. axillarts). 



plexus, and there is reason for believing that many 

 more elaborate fossil corals pass through a kind of 

 Amplexus stage. 



A fourth division of corals is termed Perforata 

 (Fig. 49.) These are the familiar twig-like, branched 



