78 ANTHOZOA. ZOANTHARIA 



vertical plates extending from the theca towards the 

 centre, and alternating in position with the mesenteries. 

 The septa are of different sizes, some reaching the centre, 

 others being shorter ; they frequently occur in series or 

 cycles, of which three or more may often be distinguished, 

 the largest being the primary (b), the others the secondary, 

 tertiary, etc. In many corals found in the Palaeozoic 

 formations one of the primary septa (the cardinal septum) 

 is much smaller than those formed after it, and consequently 

 appears, at the surface of the calyx, to lie in a pit or 

 cavity, which is called a fossula (figs. 33 a, 34 A, a). 

 Usually only one fossula is present — the cardinal fossula, 

 but sometimes another, known as the counter fossula 

 (fig. 34 A, dd) occurs on the opposite side of the coral, 

 and less commonly two others (e) called alar fossula} are 

 present one on each side of the coral. 



When the septa project upwards above the edge of 

 the theca they are said to be exsert (fig. 25). The faces 

 of the septa are sometimes 

 plane, but usually bear ridges, 

 granules, or spines. In some 

 corals the septa are poorly de- 

 veloped, and may be represented 

 by ridges only or by rows of 

 spines. In the centre of the 

 visceral chamber, where the Fig. 25. Montlivaltia trochoides, 



t , , . «, Inferior Oolite, showing exsert 



larger septa meet, there is often sep ta. x i. 



a vertical rod, which extends 



from the base of the chamber to the bottom of the calyx ; 



this is the columella (figs. 24 a ; 31 c). Its structure 



varies considerably ; when it is solid and ends in a knob 



or point in the calyx, it is said to be styliform ; sometimes 



the top is porous or spongy. When the columella consists 



