THE GRAPTOLITES AND CORALS 



307 



differs altogether from that of Zaphrentis and Lithostrotion, 

 and as it is generally characteristic of Mesozoic and 

 later corals, they are often united as Hexacoralla in con- 

 trast to the Palaeozoic Tetracoralla. 



A vertical section shows the visceral chamber to be 



FIG. 93. PARASMILIA CENTRALIS (MANTELL). 



a, Young specimen. (Xf-) b, Vertical section. (X|-) Trabecular colu- 

 mella in centre, black ; dots represent granulations on septa ; thick 

 marginal lines represent theca in section ; edges of half the septa seen 

 at the top and (broken section) at the bottom, c, Calyx. (X2.) 

 Numbers denote septa of the four cycles. (After Milne- Edwards and 

 Haime. ) 



free from tabulae and dissepiments ; the surfaces of the 

 septa bear little tubercles. At the edge of the calyx the 

 septa project and extend across the theca into continuity 

 with a series of vertical ridges (cosia] on its outer surface; 

 These costae show by their varying strength an arrange- 



