
AMONGST THE CORALS OF THE PALA OZOIC PERIOD. 249 
size, occupied by corallites which may be smaller by one half than the normal. 
In F. dubia, again, the corallites are normally of two sizes, small and large, 
the small nearly equalling the large in number, and being everywhere distributed 
amongst them. 
The remaining causes which influence the form and mode of growth amongst 
the compound Paleozoic corals are of secondary importance, and need merely 
be alluded to. The relations of the different corallites of a compound corallum 
to one another vary greatly in different species, and sometimes even in different 
individuals of the same species, and the general form of the colony necessarily 
exhibits a corresponding variability. When the corallites are in close contiguity, 
with or without absolute fusion and amalgamation of their walls, then we get 
compact corals, as in Columnaria, Favistella, Columnopora, Favosites, Cheetetes, 
- Michelinia, &c. On the other hand, when the corallites are not closely 
contiguous, then we get fasiculate corals, as in Syringopora, Diphyphyllum, &c. 
Even where the corallites are contiguous, the form of the whole corallum is 
very variable, though any given portion of it is compact; the variations 
depending on laws of which we are at present very ignorant. Such coralla are 
typically massive ; but they present themselves also in the form of dendroid 
or ramose growths, thin flattened horizontal expansions or vertical fronds, or 
sometimes as thin parasitic crusts; and all these variations may be found 
within the limits of a single genus. In certain instances, as in Favosites, the 
corallites though contiguous are not united by their walls; in other instances, 
as in Favistella, such union takes place. In other cases, as in Phillipsastrea, 
the walls of the corallites are wanting, and the different members of the colony 
are firmly united by the confluence of their septa. Finally, amongst the 
fasciculate coralla the chief differences of form are due to the comparative 
independence of the corallites (as in Diphyphyllum), or to the extent to which 
the corallites may be united by exothecal processes (Lridophyllum, Syringopora, 
Haimeophyllum, &c.) 
VOL. XXVII. PART III. oT 
