190 Prof. P. M. Duncan on the Hard Structures of 



of the lax tissue which is the first stage of exogenous growth. 

 Moreover they occasionally assist in the production of buds. 

 On the corallites the spinules are small, except where they 

 are about to enter into the formation of bads or mural 

 tissue. 



The formation of mural tissue is part of the interesting 

 exogenous growth, and it can be examined amongst the lax 

 structures at the junction of the corallites with the central or 

 axial corallite. The spinules form the props, or uprights, on 

 the top, over which a thin film of calcareous tissue grows 

 gradually, and stretches from the top of one spinule to that of 

 the others close by. Here and there the tops will be seen 

 slightly flattened, and elsewhere the flattening extends, so that 

 the tops are united by a fragile arch which unites with those 

 of others, and tends to form an irregular discontinuous 

 roofing. 



This kind of growth proceeds from and on the top of all 

 the spinules, and a space is thus covered in and a new outer 

 surface is seen. The floor of this space is the old wall with 

 its perforations, the supports are the spinules, and the roof is 

 the new growth, which is porous on account of the irregular 

 development of the films. A similar growth occurs on the 

 corallites, especially at the outer part of the margin in young 

 corallites, and all around the calice in old and apical ones ; 

 but in these instances the costje assist as well as the spinules, 

 for growths can be traced from them outwards, which become 

 the origin and sujiports of arching-over trabecute, and thin 

 filmy plates that will sooner or later produce a roof, or, in 

 other words, part of a new wall. A development of spinules 

 often takes place on the costal edge near the calicular margin, 

 and thread-like growths stretch from one to the others along 

 the costaj ; they lay the foundation of a new costa. 



Two kinds of growth appear to occur, after the develop- 

 ment of the new part of the wall : in one case spinules grow 

 from it, and costal laminge are produced in the manner just 

 mentioned, an exact repetition of the former state of things 

 coming to pass ; and in the other irregular, long, straight, 

 ragged growths occur from all parts of the new surface, and 

 combine to form a lax and porous structure. 



The amount of spinulation, or rather of true denticulation, 

 on the edges of the costal, in some parts of the colony, is so 

 great that the parts, were they detached, would be considered 

 to belong to a difi'erent variety. It is in these parts, however, 

 that gemmation is rapid and frequent, and exogenous growth 

 very decided and regular. 



When transverse sections of a branch or budding twig of a 



