15K1TIS1I I'OSSIL CORALS. 



shaped, wavy in their outhiic, nipped in 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 vice versa. In corals which are simple and horizontal 

 the wall is covered completely by the calico, and the septa arc 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, wdde, 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 coralhtes in a compomid 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 aiid the general 

 surface is by no means easy. 



Wall. — The wall gives support to the costa: externally and to the septa internally, and 

 it can be seen in the most complicated corals between the costse 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 Madrcporaria. Every 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 cxothecal structures or to the cccnenchyma as to be indistinguishable from them; 

 and in some large simple corals, where the cpitheca 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 tabulfc. 



1 Plate I, figs. 3, 1-1. = Plate III, figs. 3, 4 ; Plate IV, fig. l.s. a pi„te I, figs. 3, 14. 



* Plate IV, fig. 6. 



