INTRODUCTION. 9 



between the secondary and the fifth; then others between the third and fourth 

 and third and fifth. This regular cychcal arrangement multiphes 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 decemeraP arrangements are accounted for either 

 by the abortion or duplication of a system or by their being natural and normal 

 types. 



The palseozoic 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 Axopora 

 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 

 f(jr 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. 



3 Plate VII, figs. 11, 12, 13, 14. 



