6 BRITISH FOSSIL CORALS. 
shaped, wavy in their outline, nipped im centrally or in the figure of eight, more or less 
square, pentagonal, hexagonal, polygonal, polygonal and elongated, linear or serial, ser- 
pentine, &c. 
The margin is not always on the same plane throughout. It may be ridged, so as to 
form an ornamental series of projecting angles; the plane of the minor axis may be much 
higher than that of the major, and wce versd. In corals which are simple and horizontal 
the wall is covered completely by the calice, and the septa are necessarily very exsert. 
The calice may be prominent, and even placed at the end of a cone, or may be 
depressed below the surface, as in many compound corals. Calices may be distant or 
connected together by their walls, or they may form series by a succession of calices 
running one into the other in a linear or radiating direction. 
The opening of the calice may be very wide and everted or contracted and inverted ; 
the calice may be deep, shallow, wide, narrow, and widely open; its margin may be 
broad, flat, or narrow, and sharp; moreover, it may be below or above the bend of the 
top of the septa. Deformed calices are produced by the pressure incident to the growth 
of crowded corallites in a compound corallum, and a great number of calices are more or 
less altered in outline by the phenomena of fissiparous and calicinal reproduction. 
The calices vary in size on different parts of the same corallum. 
In some genera one half of the calicular margin may be lip-shaped or more elevated. 
than the other, and in a few the distinction between the calicular fossa and the general 
surface is by no means easy. 
Waill.—The wall gives support to the coste externally and to the septa internally, and 
it can be seen in the most complicated corals between the cost at the bottom of the 
intercostal spaces and between the septa, where it bounds externally the septal interloculi. 
It determines the shape of the corallum and the amount of its solidity; moreover, it has 
intimate relations with the columella and endotheca, as well as with the exotheca. 
The hardness and thickness of some walls’ is as remarkable as the porosity, reticulate 
character, and fragility of others, and the so-called perforate’ condition of the last is always 
noticed in an important section of the Madreporaria. Fivery possible variety of thickness 
and solidity may be noticed, as well as of fragility, thinness, and porosity ; moreover, 
these opposite conditions are brought together by the existence of perforations in compara- 
tively solid walls. 
Usually the wall is a very prominent feature in the corallum ;° but it may become so 
united to exothecal structures or to the ccenenchyma as to be indistinguishable from them ; 
and in some large simple corals, where the epitheca is strongly developed, the wall is 
either rudimentary or has become absorbed.‘ In these species the coral is kept together 
by the enormous development of the dissepiments or tabulz. 
1 Plate I, figs. 3, 14. * Plate III, figs. 3, 4; Plate IV, fig. 18. 3 Plate I, figs. 3, 14. 
4 Plate IV, fig. 6. 
