86 ANTHOZOA. ZOANTHARIA 



By many authors the Madreporarian corals have been 

 divided into three sections: (1) the Aporosa, (2) the 

 Perforata, and (3) the Rugosa. The characters of these 

 groups are : — 



(1) Aporosa. Theca and septa solid, the latter generally 

 in multiples of six, with usually six primary septa. Tabulae 

 rare. 



(2) Perforata. Distinguished from the Aporosa by 

 the septa and theca being perforate. The perforations 

 are sometimes numerous, so that the skeleton appears to 

 consist of a network of rods. 



(3) Rugosa. Septa and theca solid, tabulae well 

 developed. The coral is usually bilaterally symmetrical 

 owing to the arrangement of the septa on either side of 

 two opposite primary septa (the cardinal and counter septa), 

 and to the presence of one or more fossulae. When four 

 fossulae are present a tetrameral character is given to the 

 coral. The Rugosa are almost limited to the Palaeozoic 

 formations ; the name of the group is taken from the 

 vertical ridges often seen on the wall of the coral. 



It is doubtful if this classification can be maintained. 

 The separation of the Aporosa from the Perforata is 

 especially difficult, since some of the former possess a few 

 perforations in the septa, whilst the latter occasionally 

 have nearly compact septa. 



Until recently it has been generally maintained that 

 the Rugosa possess only four primary septa — the cardinal, 

 the counter, and two alar septa, which divide the coral into 

 quadrants ; on account of this the name Tetracoralla has 

 sometimes been used for this group. The study of the 

 development of the septa has shown that there are really 

 six primary septa, and the Rugose corals consequently 



