32 BRITISH FOSSIL CORALS, 
Well-aérated water of a certain temperature and containing minute organisms is abso- 
lutely necessary for the nutrition 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 nourishment 
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 bathymetrical 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 aérated 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 within one cavity that distinguishes true Wadreporaria from the hydroid Acalephs. 
The tabulate corals have been classified amongst these last, but upon imsufficient 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 tabulz are not necessarily calicular bases, for they may often be separated 
from the continuous septa and columelle. 
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,! 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 tabulz in the endothecas must be ascertained. 
1 Plate I, figs. 1, 2, 3, 4, 14, 15. 2 Plate III, figs. 3,4; Plate IV, fig. 1S. 
3 Plate III, figs. 9, 10, 11, 16. 
