MORPHOLOGY OF THE CORAL SKELETON. 53 



SYNAPTICULAR WALL. 



The descriptive term tells how such a wall is formed. The synapticulae 

 are iisually grouped in a definite zone, so as to form a perforate wall. 

 Stephanomorpha possesses such a wall. Fungia is a typical example. The 

 walls of the EupsammidaB are synapticulate (cf. descriptions of BalanophijUia 

 irrorata, p. 163, and Eupsammia elaborata, p. 182). It is scarcely necessary 

 to state that the synapticulate and imperforate walls grade into each other. 



COLUMELLA. 



The columella is the result of an attempt of the coral zooid to build a 

 central basal support for itself. Three different kinds may be recognized. 

 First, the false columella is formed by the fusion of the inner ends of the 

 septa into a more or less compact mass. The variation of tliis class of 

 columellse is in the degree of solidity — if the septa fuse simply by a few 

 processes without secondary thickening, the columella will be weak and 

 spongy ; if they fuse firmly by their inner margins and are then secondarily 

 thickened, a large strong columella will be the result. Second, the lamellar 

 columella is where the axial space of the corallite is occupied by a lamella. 

 Very often it can be shown that this kind of a columella is merely a differ- 

 entiated portion of some large septum, and such is almost certainly its 

 origin in all cases.' Third, the true columella is of several kinds. In a 

 considerable number of genera it is a single style that rises from the bottom 

 of the calice; in some others it consists of a varying number of rods or 

 more or less flattened and twisted pieces that have their origin at the bottom 

 of the calice. 



DISSEPIMENTS. 



Dissepiments are the thin calcareous partitions by which tlie coral 

 zooid cuts itself off" from the lower part of the corallite cavity wliich it can 

 no longer occupy. As growth proceeds along the upper portion of the 

 skeleton the zooid is continually being lifted upward, and a space below it 

 is left vacant. Dissepiments are formed to give basal support to the zooid. 

 The dissepiments are arched, the highest part usually being at tlie wall. 

 The distance apart is usually quite regular, as it represents the amount of 

 a growth period." 



' Cf. Ogilvie, op. cit., p. 245. 'Ogilvie, op. cit., p. 244. 



