236 ACTINOZOA. 



ences between the East and West Indian species 

 of Corals, or between the several Atlantic and 

 Pacific forms of the class, often curiously resem- 

 bling one another under similar conditions of 

 depth and temperature, but, in a large number of 

 cases, specifically distinct, may thus be easily 

 accounted for. Many genera of fixed Actinozoa, 

 abundant in one hemisphere, are found wholly 

 wanting in the other. To a less extent is this 

 observation true of the soft-bodied or free-swim- 

 ming species. 



Section V. 



RELATIONS OF ACTINOZOA TO TIME. 



I. General History of Actinozoa. — 2. History of Zoantharia. — 3. 

 History of Rugosa. — 4. History of Alcyonaria. — 5, Silurian 

 Corals. — 6. Devonian Corals. — 7. Carboniferous Corals. — 8. Per- 

 mian Corals. — 9. Triassic Corals. — 10. Jurassic Corals. — 11. 

 Cretaceous Corals. — 12. Tertiary Corals. — 13. Recent Actinozoa. 



I, Oeneral History of Actisiozoa* — Acti- 

 nozoa appear to have been numerous during each 

 of the greater artificial geologic epochs. The hard 

 parts of the coralligenous species only have been 

 preserved. Hence the expressions "fossil Corals" 

 and " fossil Actinozoa " may be used as syno- 

 nymous. 



One order, CtenopJiora, has no fossil represen- 

 tatives. The Rugosa, on the other hand, are 

 wholly extinct. 



The accompanying table exhibits, from a general 

 point of view, the relations to time of the prin- 

 cipal groups of Actinozoa. Lists are appended of 

 those genera of Corals Avhich range through more 

 than one geological period. 



