8 BRITISH FOSSIL CORALS. 



of structure between these and the highl}' developed septa of some Tertiary corals, where 

 the laminae 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 certain 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 III, 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 primar\', are termed the 

 secondary, and constitute a second cycle. The Alveopora has, then, two cycles of septa, or 

 six primary and six secondary. In very many species other septa are developed, which 

 are always foimd regularly distributed, one occupying each interseptal loculus. That is 

 to say, in every intei'septal loculus between the original primary and the after-coming 

 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 thu'd, 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 tertial-y constitute the/o/«V/< order of the fourth cycle, and those between the secon- 

 dary and the tertiary the fffh order of the fourth cycle. The septa arise simultaneously 

 in all the systems in this manner.^ 



The number of the septa in the last instance is forty-eight, or five orders of four 

 cycles in six systems. Each system contains the following orders : 



1st. 4th. 3rd. 5th. 2nd. 5th. 3rd. 4tli— 1st. 



Any other septa are introduced between the primary and the fourth septa, then 

 1 Plate I, fiir. 15. - Plate IX, figs. 5, C. ^ Plate V, figs. 3, 4, 9. 



