18 . jmiTISII FOSSIL CORALS. 



The synapiicidcB arc not considered to be endotliecal structv;res by MM. Milne- 

 Edwards and Haime, but their development in some species renders their present clas- 

 sification necessary. Li their feeblest development they are papillae (on opposite septal 

 lamina?), 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. 



Exotheca. — There are structures resembling endothecal dissepiments between the 

 costae of some species •} in others these sclerenehymatous laminse — the exotheca — extend 

 beyond the costae 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 costse, 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 costae 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 laminae dividing the space betweer 

 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 Turbinolice . 



In Solenastrcea they may be distinguished as forming cells on the wall and between 

 the costae and as a tissue which extends around each corallite. 



The upper surface of the dissepiments is often marked with elevations resembling 

 blunt papilla?. 



The genus Galaxea has this exothecal cell-growth in excess ; it is termed in such an 

 instance Peritheca} 



Cosnencliyma^ — 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, M'hich is very cellular in 

 its simplest development and hard and sohd in its greatest, is really an exothecal %ix\xcX\]LX&, 

 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 cccncnchyma difl'er greatly in many species. 



1 Plate I, figs. 11,18. = Plate V, figs. 2— .5. 



■ Plate I, fig. 19. * Plate IV, figs. 7, 12, 17, 18. 



