62 ACTINOZOA. 



and comparatively rare in argillaceous and arenaceous 

 beds ; this is indeed what might be expected, since 

 existing forms can, as a general rule, live only in clear 

 water ; consequently very many limestone rocks are formed 

 almost entirely of coral remains. In North America corals 

 are abundant in the Ordovician, but in England only a 

 few forms have been found; they are much more abundant 

 in the Silurian, especially in the Wenlock Limestone, the 

 chief genera being Haly sites, Omphyma, Acervularia, Fa- 

 vosites, Heliolites, and Cyathophyllum. In the Devonian, 

 the last four are still well represented ; Zaphrentis and 

 Calceola also occur. The genera Lithostrotion, Amplexus, 

 and Michelinia are confined to the Carboniferous System 

 in England, and the following do not extend beyond 

 it, Zaphrentis, Syringopora, Favosites, and Cyathophyl- 

 lum. In the Permian and Trias corals are rare. In the 

 Lias a few forms have been found, such as Montlivaltia, 

 Thamnastrcea, Thecosmilia, and Isastrcea. In the Oolites, 

 these four genera become extremely abundant in England, 

 and many others also occur. In the Cretaceous formations, 

 corals become much rarer, the chief being Holocystis, Micra- 

 bacia, and Parasmilia. Corals are also rare in the English 

 Cainozoic ; Litharcea, Dendrophyllia, and Oculina occurring 

 in the Eocene, Madrepora in the Oligocene, and Flabellum 

 in the Pliocene. 



