THE GRAPTOLITES AND CORALS 309 



stages only. Rejuvenation may also occur to lengthen 

 out the life-history, but that is an independent pheno- 

 menon that may intervene at any stage. 



The classification of the corals is in a state of flux at 

 present, and the following scheme is really out-of-date, 

 but in the absence of any complete modern scheme it 

 must be retained provisionally. Also the classification of 

 corals cannot be separated from that of their non-cal- 

 careous allies, which are here distinguished by an asterisk 

 as of little importance or none to the palaeontologist. 



CLASS : ACTINOZOA (or Anthozoa). 

 SUB-CLASS: Zoantharia. 



ORDER: ACTINIARIA* (sea-anemones). 



ANTIPATHARIA* (with horny skeleton). 



MADREPORARIA (with calcareous skeleton : the true 

 corals). 



SUB-ORDER : Rugosa. These are the Palaeozoic 

 Madreporaria, often termed Tetracoralla, because character- 

 ized by four principal septa (cardinal, counter, and two 

 alar), Mesozoic and later corals being termed Hexacoralla, 

 because their symmetry is based on six main septa. 

 (The recognition of the early development of the counter- 

 lateral septa, making six, in the Rugosa has weakened, 

 but not destroyed, this distinction.) The usual abundance 

 of tabulae is another general point of distinction in the 

 Rugosa. 



GROUP : Inexpleta. This includes a few simple 

 corals, in which the interseptal loculi are almost entirely 

 undivided, tabulae and dissepiments being very rare. 

 Common genera : TurUnolopsis, conical, with occasional 

 tabulae, found as casts in Llandovery Sandstone ; Palceo- 

 cyclus (Fig. 94, a), of which the commonest species is 

 discoidal i.e., the angle of the cone is increased to 180 



