298 Mr. J. Thomson and Dr. H. A. Nicholson on the 



cylindrical, or cylindro-conical in form. When compound, 

 the corallum is usually fasciculate ; and its mode of increase 

 is by calicular gemmation, the young corallites being always 

 produced near the periphery of the old calices (PI. XII. 

 figs. 1 & 3). 



The epitheca is thin and complete, marked with fine en- 

 circling strise and shallow accretion-ridges. The calice is 

 moderately deep, its margin being sometimes thin, at other 

 times thick and everted. 



In the centre of the visceral chamber is a small, compressed, 

 compact, or sometimes imperfectly cellular, styliform columella, 

 which fonns a small projection in the floor of the calice (Ann. 

 & Mag. Nat. Hist., Feb. 1876, PI. VIII. fig. 8). As seen in 

 longitudinal sections, the columella (PI. XII. figs. 2 A, 3 A, 

 & 6 a) forms a distinct thin line, which usually runs from the 

 bottom of the visceral chamber to the floor of the calice as a 

 continuous rod. Sometimes, however, it is absent or inter- 

 rupted over portions of its course (figs. 3a & 6 a). It was 

 at first difiicult to determine whether this was due to any 

 real want of continuity, or whether it was not caused by 

 flexures of the corallum taking the columella at places out 

 of the line of section ; but it seems to be really due to the 

 former cause. As seen in transverse sections (PI. XII. figs. 1, 

 2, 4, & 5), the columella is shown to be markedly compressed 

 laterally. 



The central area of the corallum is occjipied by tabulae, over 

 the upper surfaces of which the septa do not extend, or only 

 to a very limited extent. The tabulae are very close-set, often 

 inosculating or almost vesicular, and usually distinctly elevated 

 just before they are pierced by the columella. The result of 

 this last-mentioned peculiarity is that a transverse section cuts 

 through more than one of the tabulae in the immediate vicinity of 

 the columella. Hence in transverse sections (PI. XII. figs. 2 

 & 4) the columella is seen to be surrounded by the divided 

 edges of several of the tabulae, which might lead to the eiTO- 

 neous impression that the columella is composed of twisted 

 lamellae, though longitudinal sections clearly prove that this 

 is not the case. In no case do the tabuljE extend to the inner 

 surface of the wall, though the central area which they occupy 

 is one of very considerable dimensions. 



The septa (PI. XII. figs. 1-6) are well developed, but 

 always fall short of the columella by a considerable space. 

 Hence, though they infringe upon the margins of the tabulate 

 area, they are never continued to the centre, and they invariably 

 leave the tabulae exposed to view over a conspicuous median 

 space. A septal fossula, containing a' single short septum, is 

 often present. 



