302 PALEONTOLOGY 



The transverse sections also show that the cone is 

 hollow, consisting of a visceral cavity with a comparatively 

 thin wall (the theca), and divided by the septa into loculi. 

 Near the theca there may be seen a few curved cal- 

 careous plates crossing some of the loculi : these are 

 called dissepiments. 



A vertical section shows that the floor of the calyx is 

 the last of a series of horizontal partitions (tabula:') across 

 the visceral cavity, which mtrst be formed, as growth pro- 

 ceeds, in somewhat the same way as the septa are formed 

 in a cephalopod-shell. They do not, however, serve the 

 same function, as all corals are fixed animals; they are 

 much less regular in arrangement, and have no perfora- 

 tion like a siphuncle. Each tabula is convex upwards, 

 and in addition to the cardinal fossula there can some- 

 times be recognized three other similar, but much 

 slighter, fossulae, in the position of the two alar and 

 counter-septa. The counter-lateral septa have no 

 fossulae. The septa are continuous through the tabulae. 

 In those species ofZaphrentis which have external ribbing, 

 the ribs alternate with the septa. 



The general facts of the development of the septa 

 stated for Zaphrentis apply to a very large series of 

 Palaeozoic corals, which constitute the extinct order 

 Rugosa. By the late Professor Haeckel they were called 

 the "Tetracoralla,'' because of the four principal septa and 

 fossulae, and in distinction from the Mesozoic and modern 

 " Hexacoralla," in which the septa are arranged by sixes. 

 But the recent discovery of the early appearance of the 

 counter-lateral septa makes this numerical distinction 

 inaccurate. 



