8 BRITISH FOSSIL CORALS. 
of structure between these and the highly developed septa of some Tertiary corals, where 
the lamin composing the septa are distinct, very long, broad, and imperforate, very 
much arched and exsert, beautifully dentate on their free upper margin, and magnificently 
ornamented with granules in regular series.’ 
The number of septa in a calice varies in many species, and there is great diversity 
in their arrangement. ‘The number and arrangement of the septa differ according to the 
age and development of the individual, to a certaim extent. Many species have six septa, 
never more and never less; others have a second series, and a new septum is introduced 
between each of the old. Thus twelve septa and no more are found in a species of 
Alveopora. (Plate ILI, fig. 5.) 
The six septa which appear first of all, are termed the primary, and they constitute a 
cycle or order ; the next six, which are developed between the primary, are termed the 
secondary, and constitute a second cycle. ‘The A/veopora has, then, two cycles of septa, or 
six primary and six secondary. In very many species other septa are developed, which 
are always found regularly distributed, one occupying each interseptal loculus. That is 
to say, in every interseptal loculus between the original primary and the after-comimg 
secondary septum a third arises from the wall. There are, therefore, twelve of these 
tertiary septa, and the twelve form the third cycle or order. The three cycles, first, 
second, and third, combined, form twenty-four septa.” That is to say, between two primary 
septa there is one secondary and two tertiary septa. These septa between the two primary 
constitute a system; and when the primary septa are six in number there are six systems. 
If there be twelve septa, there are six systems of two cycles ; and if there be twenty-four, 
there are three cycles in six systems. 
There are interlocular spaces between the first septa and the tertiary, and between 
the tertiary and the secondary; any more septa must be developed one by one in these 
spaces. ‘The additional septa are, in fact, developed in the space which intervenes between 
the first and the third septa, and simultaneously others come in between the second and 
the third septa, so that in each system four more septa arise. ‘Those between the primary 
and tertiary constitute the fourth order of the fourth cycle, and those between the secon- 
dary and the tertiary the ji/th order of the fourth cycle. The septa arise simultaneously 
in all the systems in this mamner.® 
The number of the septa in the last instance is forty-eight, or five orders of four 
cycles in six systems. ach system contains the following orders : 
Ist. Ath. 3rd. 5th. 2nd. 5th. 3rd. 4th—lst. 
See ooo 
8 
Any other septa are introduced between the primary and the fourth septa, then 
1 Plate I, fig. 15. 2 Plate IX, figs. 5, 6. 3 Plate V, figs. 3, 4, 9. 
