144 PROF. p. M. DUNCAN ON THE 



respects resemble those of the large septa. There is a remark- 

 able contrast between the stoutness and solidity of the synapticula 

 and the delicacy, thinness, and often perforate and fenestrate con- 

 dition of the higher orders of septa. 



The smallest septa are nearly transparent, and consist of a 

 more solid portion, which is in contact with the synapticular 

 ridges, aud of a perforated and thin portion, which corresponds 

 with the interridge, groove, or canal. The perforations open 

 into the canals ; and thus there are more or less vertical and 

 slightly curved alternate rows of foramina and solid structure. 

 High up in the thin septa, aud just beyond the top of the synap- 

 ticula, the close part of their structure sometimes becomes almost 

 rod-like, and broad processes arch from it on either side over 

 foramina in the perforated portion. 



Eemains of this arrangement can be detected on the free edges of 

 the larger septa, and vertical lines with curved ones between them , 

 indicated by a denser molecular arrangement of the carbonate of 

 lime, are to be seen on worn edges. It is evident that the fenes- 

 trate condition of the smallest septa is not permanent, and is due 

 to incomplete development. 



A tangential section across the coral near the circumference 

 shows septa, of all sizes united by synapticula, the basal wall also 

 a synapticulate structure, and the costse below. As the section 

 is vertical and the direction of the synapticula is more or less 

 curved, they are found cut obliquely in some places, through a 

 considerable height in others, and fairly across in many spots. 



The large septa increase in width downwards to where the 

 synapticula arise, and then diminish to the wall ; and their re- 

 lation to a very slender perforated septum on either isize is seen 

 to be through the medium of short stout synapticula. At the 

 basal wall there are transverse views of large and small septa ; 

 and it is evident that, here and there, the wall is deficient, leaving 

 spaces corresponding with the openings which may be seen on 

 the underside of the coral between the costge. The costse are 

 continuous with the septa below the wall. Now the section 

 indicates that the wall is composed of more or less horizontal 

 or curved synapticula, thicker than others, above ; and where 

 they are wanting, there is an opening leading upwards into an 

 interseptal loculus, but bounded by the synapticulum above. 

 The opening leads to a canal, and by it to the free part of the 

 loculus. The openings are found near the circumference of the 



