244 TABULATE CORALS. 



opening over the whole upper surface. In other cases {e.g., H. 

 Grayi, E. and H.) the corallum is ramose or lobate, fixed by 

 its base, and having the caHces covering the whole of the free 

 surface. The internal structure of the corallum can be readily 

 studied in actual specimens or in polished sections, and little 

 fresh information is afforded by thin slices. The most impor- 

 tant points to notice, as differentiating the genus from its im- 

 mediate allies, or as otherwise of interest, are the following : — 



The corallites are universally and throughout in complete 

 contact, their walls being so entirely amalgamated that no 

 traces of their originally duplex constitution can be detected. 

 No apertures of the nature of "mural pores" are known to 

 exist in the walls. 



The walls of the larger corallites are folded so as to give rise 

 to twelve septal ridges, which are continuous from the top to 

 the bottom of the visceral chamber, and are not spiniform. In 

 some cases {e.g., H. megastoina, M'Coy) the septa are marginal 

 and rudimentary (PL XII., fig. 2) ; in other cases they extend 

 inwards for a considerable distance ; but in no case do they 

 actually meet in the centre of the visceral chamber. No septa 

 exist in the smaller corallites. 



The number of the smaller corallites varies much in different 

 species, but there is always enough of them to completely 

 isolate the larger tubes. Usually there are several layers of 

 small tubes between any given pair of the larger tubes, but 

 there are only from two to five rows in H. megastoma, M'Coy ; 

 and there are usually no more than two rows, or only a single 

 one, separating the larger corallites in H. dubia, Fr. Schmidt, 

 and In H. plasinoporoides, Nich. and Eth., jun. 



The smaller corallites of Heliolitcs are principally distin- 

 guished from the corresponding tubes in Propora and Plasmo- 

 pora by their regularly polygonal shape, and by the fact that 

 their walls are never obsolete, but are completely developed 

 (PI. XII., fig. 2 a). In long sections, therefore, no difficulty 

 is experienced in recognising the walls of the smaller tubes. 



The tabulae are well developed in both the larger and 



