32 BRITISH FOSSIL CORALS. 



Well-aeratecl water of a certain temperature and containing minute organisms is abso- 

 lutely necessary for the nntrition and respiration of corals ; and mud and sediment 

 held in suspension by brackish water, or by water very slightly saline, are very noxious. 



Corals soon die when exposed to such adverse influences ; and it is probable that the 

 contractions which are noticed on some simple forms are due to periods when noui'ishment 

 was scarce and the sea-water impure. 



Corals are often phosphorescent ; and this is very constantly observed when they have 

 been removed from the sea and allowed to drain away on stones. 



There are no special structures in the mesenteric folds which account for the process 

 of absorption, and the method of the development of the male and female elements of 

 generation in them is not satisfactorily determined. The tubular processes allow the ova 

 to escape, and the ciliary motion of the visceral cavity tends to their ejection. The gene- 

 ration of corals is said to require a temperature of not less than 75°; but it must be 

 remembered that very temperate seas have their corals, and that the coast of Norway and 

 of Scotland abounds with them. 



Without entering into the question of the geographical and bathynietrical distribution 

 of corals, it may be safely determined that the perforate corals are the most rapid growers, 

 and have the largest amount of soft tissues ; they are usually found where the sea is the 

 best aerated and full of organisms, just as some of the most solid of the aporose corals are 

 to be found in calm water and at great depth. 



It is the comprehension of the stomach, pylorus, mesenteric folds, and tubular pro- 

 cesses Avithin one cavity that distinguishes true Madreporaria from the liydroid Acalephs. 

 The tabulate corals have been classified amongst these last, but upon insufficient data. 

 Whenever the polype of a tabulate coral is proved to have its digestive and repro- 

 ductive organs in separate cavities, then the views of Agassiz will be justified, but not till 

 then ; the tabulae are not necessarily calicular bases, for they may often be separated 

 from the continuous septa and columellse. 



VI. — Classification. 



In examining a fossil coral, attention must be first of all paid to the structure of its 

 wall and septa. It must be determined whether the first is aporose,i or, on the contrary, 

 perforate,- and whether the septa are assignable to systems of cycles which follow the 

 disposition of the rugosa or not. Should there be a tubulate structure of the wall and a 

 rudimentary condition of the septa, it. should be noted. Finally, the existence of hori- 

 zontal tabulae in the endotheca^ must be ascertained. 



1 Plate I, figs. 1, 2, .3, 4, 14, 1.5. * Plate III, figs. 3, 4 ; Plate IV, fig. IS. 



3 Plrtte III, figs. 9, 10, 11, IG. 



