ANTHOZOA. ZOANTHARIA 83 



the lamellae are straight, or nearly straight, and parallel 

 with the surface, and the fibres are perpendicular ; but 

 when the surface is ridged the lamellae are curved so as to 

 be parallel with the ridges, and the fibres radiate out from 

 the dark median spots toward the curved surface of the 

 ridge (fig. 28). When the septa bear striae, granules, or 

 spines, in addition to ridges, the folding of the lamellae and 

 the radiating arrangement of the fibres become more 

 complex ; but in all cases the structure is directly related 

 to the form of the surface. The dark lines and spots 

 represent the part of the septum which was first secreted ; 

 their dark appearance may be due either to the less regular 

 arrangement of the fibres or to the imperfect calcification 

 of the material of that part. In fossil corals the dark 

 part has often undergone secondary changes which give it 

 a more distinct appearance. 



In the development of a living Zoantharian coral six 

 primary septa are first formed and appear simultaneously ; 

 next, six secondary septa are introduced between the 

 primary septa ; other cycles may subsequently be added in 

 a somewhat similar manner, so that in the adult the septa 

 have generally a completely radial arrangement. In the 

 Rugose corals of the Palaeozoic period the development 1 

 of the septa follows a different course. Instead of the six 

 primary septa appearing simultaneously, two septa (fig. 29 

 A), one on each side, are first formed and meet in the 

 centre of the coral — representing the cardinal (l) and 

 counter septa (l') of the adult, on the ventral and dorsal 

 sides respectively (fig. 34 A, a, b) ; next, two more septa 

 (fig. 29 B, 2) appear, one on each side of the cardinal 



1 This can be studied by gradually grinding down the tip of a perfect 

 specimen. The arrangement of the septa in Rugose corals can also be 

 seen either on the surface of the wall or by removing the theca. 



6—2 



