INTRODUCTION. 9 
between the secondary and the fifth; then others between the third and fourth 
and third and fifth. This regular cyclical arrangement multiplies the septa rapidly 
and regularly, and determines the symmetry of the calice and of the tentacular 
disc. When the fifth cycle is complete, there are ninety-six septa, or sixteen in each 
system.! 
When six cycles are developed, no less than 192 septa result; and seven cycles, when 
perfect, produce 384. 
It is rare for these higher cycles to be complete and the septa are aborted in many of 
the interlocular spaces. 
The primary septa are usually larger, more exsert, and extend further inwards than 
the others ; but, as the cycles become complicated, the secondary and even the tertiary septa 
often resemble the primary. Nevertheless, in the majority of instances, it is easy to 
determine the orders of the septa. The development of six systems of septa is seen in 
the majority of corals, but there are some very curious and important exceptions to its 
universality. Some species have four, five, eight or ten systems, and a corresponding 
number of large or primary septa. Moreover, monstrosities often occur, and produce an 
extra system, with a normal cyclical arrangement. 
The pentameral, octomeral, and decemeral’ arrangements are accounted for either 
by the abortion or duplication of a system or by their beimg natural and normal 
types. 
The paleozoic corals belong generally to species in which there are four primary 
septa, or in which vacant spaces produced by aborted large septa are counted with the 
other large septa. But even this generalization is not free from great exceptions, and 
there are many genera where no trace of the quaternary septal arrangement is to be 
made out. 
It must be acknowledged that septa do not always exist, and in the genus Azopora 
there is a proof of this.’ 
The septa thus elaborated as regards their succession and number present many 
peculiarities in their direction, size, length, breadth, height, exsertness, ornamentation, and 
in the structure of their lamellae and margins. They usually pass directly inwards from 
the wall towards the columella or the centre of the calicular fossa and middle of the 
visceral cavity ; occasionally they vary in this course; and it is by no means uncommon 
for the smaller septa to turn towards and even to join their larger neighbours. In calices 
where there is fissiparous growth, or the development termed serial, the septa pass inwards 
almost at right angles to the wall. 
1 Plate V, fig. 16. 
2 De Fromentel, ‘ Introduct. Polyp. Foss.,’ may be consulted concerning these unusual types; and see 
my ‘‘ Memoirs on Maltese and Australian Tertiary Corals,” ‘ Ann. Nat. Hist.,’ Sept., 1865. 
8 Plate VII, figs. 11, 12, 13, 14. 
