14 BRITISH FOSSIL CORALS. 
The septal columella: may be mistaken for the fascicular and essential ; but a longitu- 
dinal section will show that the inner edges of the septa forms the organ, and that it does 
not arise from the base. 
The parietal columella are very common, and their structure is illustrated (Plate IV, 
fig. 13; Plate VII, fig. 9). 
The calicular surface of the columella may be prominent or depressed, papillary or 
spongy ; and the organ may be very dense or consist of very lax tissue. 
The columella of the following genera may be studied with regard to this variety :— 
Parasmilia, Husmilia, Dendrosmilia, Lithophyllia, Circophylha, Rhabdophyllia, Meandrina, 
Manicina, Diploria, Heliastrea, Solenastrea, &ec. 
As a general rule, when pali exist, they are in close contact with the columella, and 
as they spring from the base they often look like lateral processes of essential columelle. 
It will be observed, in the descriptions of living corals, that the columella fills up much of 
the visceral cavity, and is developed by the inner layer of the soft tissues. Playing a very 
important part in the economy, and being in relation both with the septa and _pali, the 
columella are structures whose variations in form are of generic import. 
Coste.—The coste may be considered in a general sense to be the continuations of 
the septa beyond the wall.’ 
In some Zuréinolie the continuity between the coste and the exsert septa is very 
evident, and both of the structures are much higher than the upper margin of the wall? 
But it is very probable that this exsert condition of the septa and coste is to be referred 
to the corallum having attained its full development as regards height; the further up- 
ward growth of the wall was arrested, and only the combined costo-septal apparatus 
grew on. For when the coste of the same specimens are broken off low down, it is 
tolerably evident that the wall intervened between their bases and those of the corre- 
sponding septa. 
It would appear that the costa and septa are not developed by the same parts 
of the soft tissues except when they are exsert and above the wall; and the want 
of correspondence between the septa and costz about to be mentioned is in consequence 
of this. . 
It is probably quite correct to give the costa an origin independent of the septa, and 
to assert that they are frequently separated by the thickness of the wall from the septal 
lamine. 
The cost are developed by the inner layer of the tissue which covers the wall 
externally, and the outer surface of the wall and the exothecal structures are also formed 
by it. The costz follow, as a rule, the cyclical development of the septa, and are called 
primary, secondary, &c. 
1 Platevl fice: 2,6, 7, 11, V5, 08: 2 Plate I, figs. 6, 14, 15, 18. 
