ANTHOZOA 103 



at the present day, and their remains occur abundantly in 

 various formations in both temperate and polar regions; 

 but in the course of the later Cainozoic period the range 

 of the reef-builders became more and more restricted until 

 the present limits were reached. 



The Alcyonaria occur in all parts of the world, and are 

 found at all depths from the shore-line down to 2,300 

 fathoms, but they are most abundant at depths of less 

 than 100 fathoms ; beyond this limit the number of species 

 gradually diminishes as the depth of the water increases. 



With few exceptions the Zoantharia found in the 

 Pakeozoic formations belong to the Rugose group. The 

 Palaeozoic families which have been referred to the 

 Alcyonaria are almost unrepresented in later formations. 

 Very few of the modern Alcyonarian families occur fossil, 

 but the Pennatulidse are represented in the Trias by Pro- 

 graphularia, in the Cretaceous by Pavonaria, and in the 

 Cainozoic by Graphularia. The red coral, Corallium, is 

 found in the Cretaceous and Cainozoic (perhaps also in the 

 Jurassic) ; forms allied to Gorgonia occur in the Cretaceous 

 and Tertiary rocks ; Isis is found in the Cainozoic, and 

 perhaps also in Cretaceous formations. Spicules, similar 

 to those of Alcyonium, have been detected in the Upper 

 Cretaceous. Heliopora is first recorded from the 

 Cretaceous. The organ-pipe coral, Tuhipora, has not been 

 found fossil. 



Fossil corals are comparatively rare in argillaceous and 

 arenaceous beds but often abundant in calcareous rocks, 

 many limestones being formed almost entirely of coral 

 remains. This is indeed what might be expected, since 

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

 water. The chief features in the geological distribution 

 of the Anthozoa are given in the following table. 



