1§ BRITISH FOSSIL CORALS. 
The synapticule are not considered to be endothecal structures by MM. Milne- 
Edwards and Haime, but their development in some species renders their present clas- 
sification necessary. In their feeblest development they are papilla (on opposite septal 
laminze), which have coalesced, and thus form a bar across the interlocular space, whilst in 
their greatest they form long ridges between the septa, and they cannot be distinguished 
from very vertical dissepiments except that they do not tend to close a cavity. 
Kaotheca.—There are structures resembling endothecal dissepiments between the 
costa of some species ;' in others these sclerenchymatous laminae—the exotheca—extend 
beyond the coste and form a more or less cellular envelope to the corallite, by which it is 
joined to its fellows to form a compound corallum.? 
The simplest exothecal dissepiments are stretched horizontally across the intercostal 
spaces, they generally reach the free edge of the larger costz, and now and then hide the 
smaller. They may be inclined or not. 
The highest dissepiment, or that nearest the calice, bounds the lowest reflection of the 
soft tissues, just as the highest endothecal dissepiments bound and form the base of the 
soft tissues of the visceral cavity. 
In some species there are dissepiments between the costz very high up, and in others 
much lower down. ‘The distance between the dissepiments, their arched or plane course, 
their vesicular character, and the presence of vertical lamine dividing the space between 
dissepiments into cells, are all seen to vary greatly in different species. 
The dissepiments are very feebly developed in most simple corals, and they may be 
noticed as simple fold-like elevations on the sides of costae and as forming dimple-shaped 
depressions on the wall at the bottom of the intercostal spaces in some of the Zurbinolia. 
In Solenastrea they may be distinguished as forming cells on the wall and between 
the coste and as a tissue which extends around each corallite. 
The upper surface of the dissepiments is often marked with elevations resembling 
blunt papille. 
The genus Galazxea has this exothecal cell-growth in excess; it is termed im such an 
instance Peritheca.’ 
Cenenchyma.A—Some corallites in many compound corals are separated by a very 
dense sclerenchyma, which is variously ornamented on its free or intercalicular surface. 
In some species the walls of the corallites are evidently independent of this structure, but 
in others this is not the case. It would appear that this tissue, which is very cellular in 
its simplest development and hard and solid in its greatest, is really an ewothecal structure, 
and that it is formed by the lowest and reflected layer of the external soft tissues. ‘The 
costal markings, the granules, spines, monticules, ridges, and depressions, on the surface of 
the ccenenchyma differ greatly in many species. 
1 Plate I, figs. 11, 18. ? Plate V, figs. 2—5. 
3 Plate I, fig. 19. 4 Plate IV, figs. 7, 12, 17, 18. 
