286 Dr. H. A. Nicholson’s Contributions 
The walls of the corallites are completely amalgamated, and 
the calices are polygonal, comparatively thin-walled, and 
(including the wall) mostly about 75 inch in diameter, clusters 
of slightly larger tubes, intermixed with small tubuli, occurring 
here and there. Minute interstitial tubes are present at the 
angles of junction of many of the large corallites, or are col- 
lected into stellate groups or ‘‘ macule.” The walls of the 
corallites are marked with conspicuous periodic enlargements 
or annulations, and the tabulee are usually perforated by a 
central aperture. Mural pores have not been detected. 
As regards external characters, British examples are always 
ramose, the branches being usually compressed or flattened, 
though sometimes quite cylindrical, and often exhibiting slight 
tumid enlargements at intervals. The stems vary in diameter 
from 2 to 7 lines; andthe Arctic specimens, which I shall sub- 
sequently describe as forming a distinct variety, are still more 
massive, or even become sublobate. ‘The calices are poly- 
gonal and decidedly thin-walled (especially as compared with 
the calices of the associated Monticulipora? tumida, Phill.). 
In size they are mostly from %5 to 5 inch in diameter; but 
not uncommonly groups of corallites slightly larger than the 
average may be present here and there, and there is always a 
variable number of small interstitial tubuli. These latter are 
not only disseminated among the larger tubes, but are com- 
monly aggregated into star-shaped ‘‘ maculee”’ (Pl. X. fig. 2), 
which may be a line or more across, but are not, so far as I 
have seen, elevated above the general surface. At other 
times the tubuli occupy irregular and narrow linear tracts. 
The surface is not markedly roughened or spinulose, though 
all well-preserved examples exhibit under the microscope 
many small blunt tubercles on the lips of the calices. Lastly, 
many of the calices show the marked feature that their floor is 
formed by a tabula which is perforated by a central aperture 
(Pl. X. figs. 2 and 3). 
As regards tnternal structure, the corallites exhibit marked 
differences according as they are examined in the axial region 
or in the periphery of the branches. Thus, in the axial region 
the corallites are at first vertical, but soon bend gradually 
outwards, and they are here always polygonal and mostly 
thin-walled (Pl. X. fig. 9). The lines of demarcation between 
contiguous tubes have not yet been obliterated, and when the 
tubes are thickened (as they sometimes are) the thickening is 
uniform and not intermittent. Moreover the tabule of the 
axial region are always very sparsely developed, and, when 
present, are always imperforate and complete. | 
On the other hand, in the final or peripheral portion of — 
