304 PALEONTOLOGY 



from this tube to the outer wall there radiate a series of 

 vertical partitions (mesenteries), which also extend down 

 to the base of the general cavity, which is thus incom- 

 pletely divided into radiating chambers. It might easily 

 be supposed that the septa are the skeleton of the 

 mesenteries, but this is not the case : the mesenteries 

 occupy the centre of the interseptal loculi ; and where 

 the septa come the wall and floor of the polyp are, as it 

 were, indented by them, for the whole skeleton is a 

 secretion of the external layer of the polyp. 



2. Lithostrotion irregulare (Fig. 92) is a coral very 

 abundant in some of the uppermost beds of the Carbon- 

 iferous Limestone (zone D2). At Wrington, Somerset, 

 for instance, it occurs in great masses, red-stained by 

 iron oxide. It is a compound coral, consisting of a mass 

 of vertical cylindrical corallites (each secreted by a single 

 polyp), loosely in contact or slightly separated, and 

 originating one from another by lateral branching. Only 

 near the base of the corallum (as the whole compound 

 skeleton is termed) is branching abundant, but it occurs 

 occasionally at higher levels. Externally each corallite 

 is marked by slight horizontal wrinkles, closely set. The 

 calyx of a full-grown specimen shows twenty-four major 

 septa and as many minor, the former reaching nearly to 

 the centre, the latter scarcely projecting through a narrow 

 marginal zone of dissepiments ; but in young specimens 

 or cross-sections of the early parts of the colony, eighteen 

 major only can be found, and there may be fewer. In 

 the centre of the calyx there projects a laterally 

 compressed structure, the columella. In a vertical 



