SUBDIVISIONS. 93 



oval, in most forms, and are Isolated in position, while their 

 cavities are intersected by few and remote horizontal tabulae, 

 these structures being occasionally partially absent. The 

 smaller corallites are distinctly angular in form, and surround 

 the larger tubes completely, there being sometimes one and 

 sometimes two rows between any given pair of the latter. The 

 smaller corallites are furnished with numerous and close-set 

 tabulae, and may or may not be bounded by complete walls. 

 In the former case the tabulae are complete and horizontal, 

 whereas in the latter case they anastomose with one another, 

 and give rise to a tissue of convex lenticular vesicles (fig. i6, b), 

 the variations observable in this respect being precisely parallel 

 with those which obtain respectively in Heliolitcs and Propora. 

 The walls of the corallites of both sets are thin, and are not 

 conspicuously thickened towards their mouths. It should also 

 be borne in mind that the interstitial and closely tabulate coral- 

 lites are sometimes equal in size to the round and remotely 

 tabulate corallites which they surround ; so that in speaking of 

 the former as the " small " corallites, we only employ this term 

 in a conventional sense, as indicating their homology with the 

 small tubes of the JMonticuliporoids generally. The corallum 

 in FistiUipora often exhibits " maculae " or definite areas occu- 

 pied by the smaller corallites only. These maculae may be 

 stellate in shape ; but they are never elevated above the sur- 

 face or surrounded by radiating elevated ridges, as is the case 

 in Constellaria. Lastly, while the large rounded tubes always 

 open the surface by open calices, the intermediate angular tubes 

 often have their mouths closed in the adult condition by a thin 

 calcareous membrane. In no case are septa present. 



From the Hdiolitida:, Fistulipora is separated mainly by the 

 total absence of septa in the latter, so far as certainly known ; 

 but, as has been pointed out, there is a striking resemblance 

 in general structure between certain species of Fistulipora on 

 the one hand, and the species of Propora and Heliolites on 

 the other hand. "Mural pores" have not been detected in 

 any typicg-l form of Fistulipora ; but Dr Rominger has shown 



